I have been noticing that when I am stopped and let off the brake there is a clunk in the rear - almost as if the car behind you has bumped you - I took the truck to the dealer and there is a TSB for this issue (06086100).

They installed insulators to the yoke/body mounts to cure this issue. I cant find the TSB online but if you have a 2008 GMC sierra you may want to call your local dealer and have it looked at.