You have to determine if this is a brake issue or rear end issue, remove the rear drums and inspect for broken springs or clips, worn shoes, grooves in the drum, if that is determinded it could be the positraction clutches in the rear end differential, the remedy for the "clacking" of the rear end is to drain the fluid and refill with rear end fluid from the GM dealer and the must additive is the rear end additive conditioner that the GM dealer only sells, all other brands do not work, let us know what you find.