The brakes took a lot of effort to get the truck to stop even at slower speeds. In thinking about it now, I believe that if the rotors had been turned before then that must have also reduced the stopping capability.
Have you torn into the rear brakes yet ?
When (not IF) the adjusters freeze up , you lose a lot of braking capacity.
Pull the rears apart , free up everything , and adjust them up .
You'll notice a big difference in the pedal .