When I first purchased the truck, it ran fine, but I noticed a dull groan coming from the truck, which I assumed was a wheel bearing. Took it into a dealership, and they said it was coming from the rear end.
Contacted original dealership (truck was purchased 2 weeks prior), and they fixed the right rear axle bearing. Picked up the truck, and the same sound was there. They diagnosed it as an out of round axle housing, which caused the bearing to disintegrate and leak fluid within 2 days after the repair.
Anyways, here's what's gone wrong since they replaced that rear end with a used "certified" rear end from a wrecker;
2 - 3 shift clunks and downshift clunks.
Excessive drive shaft play.
Vibration from at speeds 110km/h - 120km/h.
The dealership rotated the drive shaft 180 degrees, balanced the drive shaft, and replaced both front and rear u-joints.
All clunks and vibrations are still present - now the dealership is good for trying to repair the issue, but they had the truck for more than half the time in the first 5 weeks of ownership. They also will let me bring the truck back, but at a loss of $1500 for depreciation.
Anyways, regarding the Avalanche or should I say GM in general, any drive shaft back lash/play including 1 -2 or 2 -3 shift clunks is considered normal by GM. There are TSB's that advise dealerships not to do anything about it.
I love the truck, but hate the problems associated with them. Great truck if you can afford gas, because they are quite thirsty (considering it is built off the Suburban chassis).
I wouldn't recommend a GM vehicle to anyone, since they cover their butts with their TSB's. I thought Chrysler was bad, but GM is actually worse. And this issue is common to all GM trucks with the 4.8/5.3 litres.