I've got a funky vibration between 45 and 65 mph but only when under heavy throttle. This is only in 4x4. I've lifted the truck about 1.5" by the torsion bars and I used shock extensions. I did notice that I've gotten behind on rotating the tires and the backs are noticably worn more than the fronts. I tried just rotating the tires on one side to put the mismatch from side to side so the diff could compensate. This changes it a bit but just barely. I then when't ahead and finished rotating the tires. The vibration got better yet but it's still very noticable through the steering wheel.

The next step is to lower the truck back to stock ride height and see if that helps. Has anyone had similar problems? To me the only way to get a vibration only under throttle is to have the front diff rotate with heavy loads putting the front U-joints at a steep angle.

Anyone have a clue what's wrong? I hate to take it to a dealer if it's something simple. Plus with the way GM is with modifications I'm sure they'll blame it on the shock extensions which I'm in no mood to take off just for a visit.

i would get your tires balanced. I cranked my t- bars up and have no vibrations. Or maybe it is the difference in tire wear and running in 4 wd?

...between 45 and 65 mph ... under heavy throttle ... in 4x4.


I'm sorry, but I've got to file this one under "why I don't buy used vehicles.":)

I'm sorry, but I've got to file this one under "why I don't buy used vehicles.":)

Why? There are guys who drag these things up to 100mph, all WOT in 4x4 with no issues.

It does seem tire related since when you moved them around it helped, even if just a little. Balancing is cheap enough to try and eliminate the possibility.

Why? There are guys who drag these things up to 100mph, all WOT in 4x4 with no issues.


OK, I may be wrong here, but my understanding of the 4wd system in the GM trucks is that they don't have a center differential, meaning that a front and rear wheel HAVE to rotate at the same speed. AWD vehicles have a center differential so the front and rear wheels can rotate at different speeds which is why they're just fine on dry pavement. Without that center differential, if the wheels can't slip you're causing a lot of stresses in the drive train unless your tires are exactly the same diameter and you keep your vehicle going exactly straight. Yes, odds are you can get away with it. But should you???

If I am wrong here, I hope someone will educate me.

The problem is most likley caused from steep cv angles, due to you turning up the Tbars. Lower the Tbars back down, and I bet it goes away.

OK, I may be wrong here, but my understanding of the 4wd system in the GM trucks is that they don't have a center differential, meaning that a front and rear wheel HAVE to rotate at the same speed. AWD vehicles have a center differential so the front and rear wheels can rotate at different speeds which is why they're just fine on dry pavement. Without that center differential, if the wheels can't slip you're causing a lot of stresses in the drive train unless your tires are exactly the same diameter and you keep your vehicle going exactly straight. Yes, odds are you can get away with it. But should you???

If I am wrong here, I hope someone will educate me.It won't hurt these trucks to race them in 4wd on the pavement. I run 10's in 4wd on pavement with my 140,000 mile 1/2 ton. As long as the T bars are down, and you have Tie rod sleeves I have never seen a problem from it. If you are a real savage you will need a billet center link, or brace.

It won't hurt these trucks to race them in 4wd on the pavement. I run 10's in 4wd on pavement with my 140,000 mile 1/2 ton. As long as the T bars are down, and you have Tie rod sleeves I have never seen a problem from it. If you are a real savage you will need a billet center link, or brace.

I still stand by my original comment, this is why I don't buy used vehicles. ;)

You are NOT going to hurt any 4wd system by running it at highway speeds.....period. You can be scared if you want but it's unwarranted.

I agree the vibration would make sense from the steeper CV angles....but I'm no where near as high as most guys on here and none of them have complained about vibrations. I would think if this was the cause it would do it all the time not just under throttle.

I highly doubt it's a tire balance problem....actually I KNOW it's not a tire balance problem because it doesn't do it all the time and it only does it under throttle, both of which rule out the balance.

I double checked the front diff mounts and they look to be ok, the front drive shaft universals also look good.

Next step......lowering the truck and see what happens.

You are NOT going to hurt any 4wd system by running it at highway speeds.....period. You can be scared if you want but it's unwarranted.


Didn't say, or at least didn't mean to imply, it was a problem running at highway speeds. Running on dry pavement at highway speeds is another story. As long as you're going in a straight line, it's not too horrible.

Check any manufacturers recommendations on running part time 4wd on dry pavement. You'll see everyone advises against it.

Edit: I do agree with your diagnosis it isn't tire balance. That would probably show up in 2wd as well. For the record, I'm also not saying 4x4 on dry pavement is what's causing your vibration. Just I wouldn't do it to my truck.

The way I look at it there is no where near enough force on the drivetrain to hurt anything what so ever. I can drop my truck in neutral at 65 and it takes about a mile for it to drift to a stop. If it was binding enough to put a strain on anything it would stop much quicker than that. The entire drivetrain is built to take 4 low with 650 ft/lb's of torque running through it.......slight binding from 1/4" of tire size mismatch isn't going to hurt anything but your mileage. You'll probably cringe of the thought of me doing 4x4 boosted launches and pulling my 14,500 lb. camper around on pavement, in 4 low at that.

I don't want to get too far off topic, so we'll just agree to to disagree. Just make sure you never buy a desert brown 2500 from a dude in WV. :D

I don't want to get too far off topic, so we'll just agree to to disagree.

Yeah, sorry. Didn't mean to hijack the thread. One thing we can agree on, even though I wouldn't drive my truck like that, this is your truck and this is a free country so I'll defend your right to drive your truck however you want.

Just make sure you never buy a desert brown 2500 from a dude in WV. :D

:rotflmao:Thanks for the heads up. :beerchug:

My LBZ will vibrate with the bars up an 1.5'' over stock. I was towing a trailer through the mountians in a late may snowstorm. Had to run 4x4 on the highway for ~100mi, vibration was terrable pulling up hill. Doesnt do it when set at stock height.

Ok now we're getting somewhere, was your vibration like mine and only under throttle?

I'm a bit busy this week with chores for the wifey, so I'm going to try lowering it on friday. If it is the CV's then it's because the front end rises under throttle putting them at an even steeper angle. I say below 45 and I'm not going fast enough to feel it, above 60 and the front end isn't up high enough to feel it.......

Any one know of a way to drop the front diff with the stock suspension?

Ok now we're getting somewhere, was your vibration like mine and only under throttle?

I'm a bit busy this week with chores for the wifey, so I'm going to try lowering it on friday. If it is the CV's then it's because the front end rises under throttle putting them at an even steeper angle. I say below 45 and I'm not going fast enough to feel it, above 60 and the front end isn't up high enough to feel it.......

Any one know of a way to drop the front diff with the stock suspension?Sounds the same mine will only vibrate under power too. With the Tbars up it will still vibrate without the trailer, just not as bad. With em down I can run 80 with the trailer with no problem, It's so smooth I sometimes forget to take it out of 4. Only way I know of to drop the IFS is to get a 4'' lift and turn it down.

Great thread. I was wondering the same thing too. I have my T-Bars cranked, and when i want to launch off the line at a traffic light or stop when its raining i put it in 4wd. I also get the vibration in the steering wheel only when under heavy acceleration.

It sounds like the crank of the T-Bars is what is causing it. I'm currently running stock tire size that i rotate fequently. Other than the t bars cranked im stock as far as lift goes. Other mods are in the sig.

keep a close eye on u joints front shaft and rear becuase if one goes left untreated you WILL be buying an extension housng on your trans!:eek: as i did!