if your truck has disc brakes on the back you have shoes inside the rear drums , rarely wear out , your prob, if you live in an area prone to rust or salt damage , pull the rear calipers and the rear brake rotor and have some watch the shoes as you or someone pulls up on the brake handle and see if the shoes move outward these are all mechanical, you may have rusted up rear cables and only pulling against the inner cables them selves , not uncommon at all , even the same thing can happen on the drum brakes , even though they use the same shoes for parking as the service brakes , they can rust up and still work the service brakes and the cables will prevent the parking brakes from working Jerry