A loose sound in the rear axlel, not like axlels catching up with each other in a limited slip rear end. The dealer just finished changing the fluid, clutches and clutch plates. They also changed the fluid to another kind, said this was the cause of the problem. The noise is more like end play and is still there. When you drive slow over uneven terrain, you can hear the noise like bumping from side to side. The truck has 22,000 miles and under warranty, but if this is something I have to live with,,,,,I'm not happy. The truck didn't make this noise until it had 20,000. The sound goes away at higher speeds and is there when you are driving straight over rough terrain.

Are you sure the spare tire is cranked up tightly?
What about a tweaked exhaust?

Checked spare, it is tight and muffler seems to be snug. You can roll the window down and hear the sound coming from the left rear dead in the center of the wheel. Sounds almost like a bad grommet in a shock, but shocks seem to be tight.

They also changed the fluid to another kind, said this was the cause of the problem.

My first thought was "Yeah another kind of fluid, as in any lube at all." With their history of underfilling or leaving them dry...just a thought.

What kind of truck is it? Is it a G80 Eaton locker? Look in your glove box bottom for the build codes and if it says G80, you don't have clutches and clutch plates. The lube for a G80 from GM is (approximately) 4 qts of "grape juice" synthetic axle lubricant. Could be a bad outer bearing or actually might be a bad u-joint.

The truck is a Sierra Classic 4/4 Allison 2007 2500/duramax HD and it is a G80 . Repair sheet says Flushed rear differential unit for clutch plates causing chatter for Doc 2063226 F2240 plates, limited slip clutch-replace part#89021677 count 4. Change fluid to synthetic.

IIRC (and I believe I do), there are no clutches in a G80. WTF?

Doesn't mean some tech / writer didn't put it on there...there's lots of confusion in that regard.

How long before you ask for an entire rear end replaced?

aRE YOU SHURE IT IS NOT YOUR WEAR PADS ON THE LEAF SPRINGS

FOUND THE PROBLEM.....The spring shackles at the end of the left leaf spring were loose causing the bump from side to side. The overload spring pad was also bumping causing the shackle to lift just enough up and down to cause a bumping sound that sounded just like the sound was coming from inside the rear end. Thanks to my son hanging over the side of the truck while I drove over some rough terrain..,I guess this was to much to ask from the dealer, the noise is gone, I bet this problem isn't new just not found. Thanks for the input...

Good to know it is fixt