Are your brakes dragging? Jack up the rear wheels, put the transmission in neutral, and see if the rear wheels turn freely. If they turn with minimal effort, then it's probably your master cylinder or the load sensing proportioning valve. Check the valve first, it's located on the body above the rear axle pumpkin. Make sure that the spring that attaches from the lever on the valve to the bracket on the axle is intact. If that spring is broken or not attached, the lspv will think that you are carrying a heavy load & send more brake fluid pressure to the rear brakes, causing them to wear out faster. I would also take off the spring (if it's there) and manually move the lever on the valve to make sure it's not stuck in one position. If the proportioning valve seems to be OK, then chances are it's your master cylinder passing fluid internally. I would also try bleeding your brakes before replacing anything, because it's cheap to do and you might just have some air in the system that's causing your problem.