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.
does you truck have drum brakes in the back
The brake shoes may be out of adjustment causing the shoes to contact the drums prematurely. You could have a proportiong valve issue, but try adjusting the rear brakes first.
The rear brakes will lock when there isn't enough weight in the rear of the vehicle, the surface is loose (like a gravel road), or they [the brakes] are applied to heavily. Newer vehicles with anti-lock brake systems (ABS) should not have this problem.
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.
Most likely brake master cylinder.
Improperly adjusted, new pads, low traction, etc.... most vans only had rear-wheel antilock brakes.
Frozen caliper? if disc brakes Broken or disconnect return spring if drum brakes? Brakes not adjusted properly? Parking brake cable frozen and not releasing?
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.
Are the brakes right and is the e-brake not sticking?
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.
warped drum. worn out brake shoes.
make sure your emergency brake is not on