In addition to all the great info from Mr. Spears. With the truck up on stands again, pull both rear wheels and drums. Then test drive the rear axle again. Except don't try shifting from forward to reverse with the axle spinning. Try one direction and observe both axles then shut off the engine. Restart in Park/neutral and then try the opposite direction, again, observe the axle rotation both sides. This will eliminate the brake topics completely! If both sides rotate (without noises) then concentrate on the brake systems operation.