Back drum brakes lock up when I barely tap the.

Most common cause is a rear parking brake cable rusted partially-applied. Gear lube leaking from an axle seal will do that too. There are other things that can cause this after doing a brake job.

Also look for a height-sensing proportioning valve near the rear axle or one of the rear wheels. It's job is to provide more brake pressure to the rear brakes when the back of the truck is heavily-loaded. If that link between the frame and axle is disconnected or broken, that valve could be stuck in the "loaded" position.