I know this is a bad sign, but I noticed lately that I get a louder than I would like 'clunk' when I shift to D or R. I replaced all 4 U-joints and found a little more slack than I was expecting in the rear end. I checked the fluid level and it was low, but instead of topping it off with 80W-90 gear oil (which is what its supposed to take right?) I added a whole bottle of 90W-140. Noise is gone for now atleast and she drives great.

My question is, will running a higher viscosity oil and overfilling the third member a bit cause any problems I should be concerned about?
If your wondering how I overfilled it, I pushed it through the stem for the bleeder hose.
Thanks.

Well I'm no machanic but I would think it could be hard on your axel and main seals. I just had my rear axel seals replaced on warranty and by time all was said and done they said it was about an 800 dollar job counting rear brakes rental car etc. I read about others putting in the 90-140 but you might want to try draining and replacing rather then overfilling - to much is generally as bad as low.......

if you are talking about your 94 blazer with a 10 bolt rearend,they are prone to the spider gears and the carrier getting excessive wear and the only solution is to replace the carrier.sometimes it is just the spider gears and you can just replace those

Yeah, I already had the spider gears, ring and pinion explode on it once, and I replaced the whole rearend.

The garage that replaced it said they had never seen one destroy itself like this one did. And they said it still had plenty of fluid in it.

Is there anyway to prevent this from happening again? I really don't want to change the gear ratio, because I would have to change the front one too. I really like the tall gears, great for mileage too.

Would a limited slip setup be better?

I had the factory 10 bolt gov-lock go out about 5 years ago after some long grades coming into Boone, N.C. Had it rebuilt there with an open carrier to save money and expedite my return home from "vacation". About a year ago the axle tube weld on the driver's side popped out and the pressed in tube would not stop leaking even after I re-welded the hole. Apparently the tube had shifted and instead of welding the tubes all the way around I searched down and found the axle all 1500's with the 6.5 should have been built with. It's a semi-floater 6 lug with a 14 bolt center housing. It came with 3.73 gears and the gov-lock posi. It was available in late 90's 2wd light 2500's and I believe some of those 1500/2500 questionable suburbans. Swapped out easily, I was able to use the same drive shaft by using a Neapco adapter U-joint. I'll have to dig the part # up. 1 year later and loving it. Besides some obvious loss of ground clearance in the middle, the bigger gov-lock is way more predictable in mud/sand and I'm not so worried about tearing up another POS 10 bolt.

Hmmm, I like that, whats the stock gear ratio in our blazers? 3.42 or 3.73?
As I said, I would like one thats stronger, but I don't want to have to change the ratio in the front axle.

I finally got some time to work on the truck some more on Saturday, so I pulled the rear dif cover and drained the fluid out.

The fluid that came out was awful, LOTS or metal stuck to the magnet, and the fluid was greenish bronze colored....YUCK.

I did however make one good discovery, I have a limited slip rearend!!!! I had the whole rearend replaced a year or so ago, and this is the first time I've really looked at it. I don't know why they wouldn't tell me they were putting a limited slip in, but by the looks of the fluid inside, I think I got scammed and didn't get a 'rebuilt' rearend, but just a used one instead. There is no way the fluid got that ugly after only such a short period of time right?

I bought 4qts of 80-140 weight gear oil, it took only about 2½ to fill to the drain plug.

Will this heavy oil cause any problems for the limited slip?

What about in the winter time when it gets to sub 0F outside for several weeks in a row?

If you have the limited slip rear you need to use a diff fluid especially made for posi units. There is also a GM additive that made for their "gov-lock" diffs. The techs call it sperm oil because that's what it looks like -:t . After the trials I've had with rears I run Amsoil synthetic posi traction diff lube and the GM additive to be sure. My blazer came with the 3.73's but I'm not sure if they all did or not. I do know that all 14 bolt semi floaters did come, stock that is, with the 3.73's which made my swap all that much easier. I did it shadetree style with jack stands, plywood, and a floor jack.

I preffer to use synthetic oil in the rear with the gov lock. Additive is not required with the synthetic but is required with other oils. Be sure that the oil you used is gl-5 rated for limited slips.;) The best fix is to swap in a 6 lug 14 bolt rear end, much stronger than a 10 bolt for a heavy rig like your blazer especially with the torque of the 6.5. I run 10 bolts in my CJ7 with lock rite lockers front and rear and haven't had any problems yet, as long as you don't call twisted splines on the short side front axle a problem):h hasn't broke yet though.

Hmmmm...I'll look around for some of that additive, do I have to get it from the dealership? Surely parts house somewhere has some?