I had the big one ton drums on my Jimmy for a while...way too much braking without some sort of adjustable prop valve. The rears would lock with moderate pressure and the fronts never would (D60 front, new brakes all four corners)
I swapped to the rear disc's with D44 calipers on the rear. Now I get no premature lock-up with moderate pedal pressure, but if I hit em harder all four corners lock-up at the same time. That's on 39.5" TSL's too...
I don't feel like I ever need more braking.
Your pick-up should have a self adjusting and load sensitive prop valve attached between the 14 bolt and the frame with an arm of some sort. The more weight in the bed, the more braking the rear will see. IMO, if you tow or haul heavy junk this is a great feature. You may be able to keep the prop valve back there and adjust it as needed. If the rears are locking up too quickly, shorten the arm. Not locking up early enough then lengthen the arm.
Rene