clunk from rear - only when releasing brakes going downhill

[Dusty]

I have heard (and felt) a clunking coming from the rear. this occurs when I am completely stopped and then release my foot off of the brake pedal when going downhill. This does not happen going uphill or over bumps. I don't know where to start to look - with the brakes or suspension. Wouldn't problems with rear crossmember, exhaust, or bottom sway bar bushings make a noise going over bumps? It actually feels like the rear brakes are sticking for a split second but this doesn't happen on level ground. Ideas???