I'm new to both this forum and Duramax/Chev. I just bought an 02 K3500 CC,LB,w/36K miles. I have not noticed such a rear end noise, clunk, or gear slack. The only time anything similar happens is when shifting the Allison into gear, then it's not a clunk, just a lurch against the brakes. Differential gear setup is exacting and if not performed with precision, either the gears will whine, clunk, growl, well you get the idea. Kinda like never buy a car built on Friday or Monday thing- well perhaps you guys are too young to remember those days.