sticky brakes

I recently bought a 2004 chevy 1500 convertion van, with four wheel disc and anti-lock brakes. I have been having a problem with the rear brakes locking up when i stop. Not accually locking but sticking. When i begin to roll, i get thump in the rear. I took this problem to the dealer and they checked everthing out. Told me there is nothing wrong, everthing is working fine. (NOT) So as far as thier concernd, that's that. I'm not too happy with that. With a 40,000 price tag, i don't expect the van to go bump at a light. Has anyone had a similar problem and if so what was done about it?? Any info would be helpful.