Driveline or brake clunk

[Rolandd]

I have a 05 Z71 SCLB. This morning after a hard stop (I really should have hit the Camry) I started without coming to a full stop. The thump that emenated from someplace in the back of the truck made me think my (nonexistant) load had slammed into the gate. Since then I have noticed that if I get on the brakes but do not come to a full stop and then restart I get a clunk proportional to how hard I slowed down. I can recreate this condition creeping in traffic or elsewhere. Yet if I shift from Drive to Reverse the driveline is tight.

 

I am thinking that either the rear drum brake shoes are snapping back on their springs or that something in the guts of the tranny is not responding well. While I have had plenty of sloppy ring and pinion sets with 150,k+ miles, with 10,k miles I expect the driveline to be flawless.