If you've gotten oil on the brake shoes it tends to burn in, making them sticky and quite prone to locking. Replacement is necessary, and you need to very thoroughly clean the metal drum as well.

The load sensing proportioning valve (LSVP) is on the right frame rail, and is connected to the rear axle with a rod. When working correctly, it balances the rear braking to the load in the bed. If your truck is lifted and it wasn't adjusted, you can have problems with lack of rear braking. Typically they don't go out of adjustment.

However, LSVP's do get gunked up and quit working, prematurely locking up the rear wheels. Removal, disassembly and cleaning restores them to full functionality.