Hy John Hill, rear wheel lockup on hard braking has been a problem for eons on pickup trucks, the weight shifts to the front, actually removing some of what little weight is over the rear wheels of an unladen pickup truck. Many cures have been tried, including an adjustable proportioning valve mounted between the frame and rear axle, the more weight in the box the more rear brakes you get. If the problem isn't causing you to lose control, and only occurs on hard braking, you could live with it, or see if you can mount an adjustable proportioning valve on your truck, like the one I described above, hope that helps P.S. the problem will be magnafied by slippery conditions.
