1992 gmc 1500 long bed with 305 2 wheel drive front disc brakes rear drum. I just got this truck and every time I come to a complete stop then let of my brakes I hear and feel a clunk like the rear brakes are sticking. I have already checked my u-joints and drive line but there tight. I don't even have to accelerate just let of the brakes and idle and before it starts moving there is a clunk like jurk in the rear of the truck that you can feel.

Do you have an intermediate driveshaft support bearing that's bad?

Uneven brake effort from front to rear can cause rear axle "roll-up" or "rotation" so when you release the brakes, the rear axle is then free to rotate back to a relaxed position. This anomaly is also prominent in my Toyota PreRunner though on that truck I can fell the release through the shift handle.
If this is your case, you can try several things to reduce it but I'm aware of it and just live with it since it's not too extreme.
If I had to do something, I'd start with a complete brake fluid flush followed by replacing front and rear brake linings with new, original equipment linings, making certain that the rotors and drums are within wear specs and that all mounting and adjusting hardware was functional and in place.