I have a 2002 Silverado 2500 HD with 130k miles. No serious problems, but one annoying issue. From a stand still when I accelerate it feels as if either the motor mounts are loose/worn or the transmission mounts are loose/worn...kind of a "clunk" feel or if something is loose.

 

Did you check your u-joints?

 

check the motor mounts, your trans mount, and u-joints. try and move them around to see if their moving. or, if you can, take a wrench/socket to the motor and tranny mounts to see if their loose.