The next question is how much flow of brake fluid you get from each caliper when you are bleeding the brakes. Do you get about the same volume of fluid at each bleeder screw when you are bleeding them?
If all that looks good, jack up the truck and get all four wheels off the ground. With the engine off, have a helper apply the brakes until you can't turn the front tires by hand and then have your helper hold the pedal in that position. Then go an try to turn the rears and see what happens. If the rear wheels will still turn, have your helper press harder to see if they can lock them up. Sometimes I will take off the tires and use a bar to turn the wheels. If the calipers are way out of adjustment it will show up with this simple test.
If the rear tires lock up first we can address that issue too.
Did you try locking up the brakes in gravel?
Let me know what you find...