I posted about noises from the rear end of my C-6 coupe. I finally took it to the dealer and took the tech for a ride around and around in the parking lot. He could hear the noise but wasn't convinced it was from the rear. When we got back inside the service manager had pulled up a bulletin from GM that described the problem exactly.
Condition/Concern: Rear axle chatter or shudder on turns on Chevrolet Corvette and Cadillac XLR
They drained and flushed the rear axle and filled it with synthetic axle lubricant GM P/N 12378261 and friction modifier additive P/N 01052358.
This was 2 days ago and the problem is now completely gone. The bulletin stated that if this didn't fix it then they should replace the right and left rear axle clutch packs. I just hope I don't have any more problems with this. The car only has 4500 miles and is just 18 months old.
They gave me a copy of the bulletin and I have posted it as a jpeg file on my pictures page at Yahoo. If you are interested you can read it by clicking on the link below. It is 2 pages long.