If you have the tow package with the self leveling shocks it is a commonly known problem. Search for 'clunk' & 'rear'.
Bad news even with replacing the shocks they noise (rattle or clunking) comes back. There are several including myself who have the third or fourth pair of shocks in. There is one TSB out supposed to fix the self leveling shock problem by replacing them. But the version of part they mention (...AC) has been replaced already by a new version (...AD) and rumors are that an AF version is on its way.
Press thumbs that your problems are a different one.
2012 Limited V6 4x4
Tow Package, Skid Plates, Tow Hooks, Tubular Side Steps, Mud Flaps, Underbody Protection
Planned: headlight washing system, amber rear blinker, rear fog light, change amber parking lights to whites, lockpick, front camera, swap RBZ to RHW