does you truck have drum brakes in the back
Your rear brakes (drum) are probably so worn out that the shoes are trying to rotate with the drum and lock up. There is an adjuster inside the drum & you might can tighten the shoes, but if they are that worn out, then replace the brake shoes.
Rear drum brakes, that lock up during heartbreaking quicker than the front brakes, can be adjusted. The rear brake pads need to be backed off.
Assuming they are Drum brakes most likely the wheel cylinder is bad.
No, they are inferior to disc brakes. Drum brakes were used from the beginning of the auto up until the late 60s. Drum brakes are more prone to overheating than disc brakes. Disc brakes also shed water much better than drum brakes which improves stopping distance in wet conditions. Disc brakes apply pressure more evenly than drum brakes thus improving stopping distance. Disc brakes are superior in every way.
Frozen caliper? if disc brakes Broken or disconnect return spring if drum brakes? Brakes not adjusted properly? Parking brake cable frozen and not releasing?
warped drum. worn out brake shoes.
If the rear brakes are drum brakes, then there could be a leaky wheel cylinder. The wetness inside the drum will cover all the brake shoes and parts and cause the wheel to lock up when applied. The build of pressure could be due to the wheel cylinder leaking and causing a pressure loss, but rising when the brakes are applied or pumped up. This may/may not be your case, but definitely worth checking the rears.
Either your emergency brake is hanging up or your wheel cylinder is leaking fluid and needs replaced if drum brakes and if disc brakes the caliper could be bad.
Wheel cylinder (drum brakes) or caliber (disc brakes) sticking. If you find this to be the case you need to replace both sides and replace your brake fluid which is contaminated with moisture.
Slam on your brakes. If your brakes lock up, then you do not have ABS. If your brakes do not lock up and you hear a knocking type of noise from your brakes, then you have ABS.
most likely if you have drum brakes. the shoe has came off the metal mount, and lodged itself in the drum. get new brake shoes. but have fun getting the drum off!!!!!!