Brakes? Ujoints?

I have a 95 chevy reg cab truck. Last night I was backing out of my driveway when the wheels suddenly locked up on me. I engaged my clutch, and put my foot on the brake and the truck was good to roll another 10 or so feet until locking up again. Does anyone have any clue what this could be. The truck has NEVER done this before. Thanks in advance for the help everyone.

Could be the brakes, a u-joint, something in the rear diff., or transmission/ transfercase. You will just have to start looking. Jack the rear wheels up off the ground and see if you can get it to catch and see if you can tell what is causing the problem. If it is not the brakes or the u-joints then disconnect the drive line (mark where it is mounted first), then spin the wheels to see if the catch is in the rear diff. Spin the drive line with the trany in neutral and see if the catch is in the trany/transfercase. Hope this helps.

I put it on stands today and pulled the drums off. My wheel cylinder looked bad so I began removing my brake pads. As I pulled the assembly from the cylinder the boot and piston pushed out of the cylinder. After that, I noticed that a chunk was missing off of my pad. It was lodged between the diff plate and cylinder. Bad stuff happening back there, but I bought new drums, cylinders, and plates. I am good to go now. Thanks for the help everyone.