I once had a 90 2500 2wd that was referred to as a "light duty" 3/4 ton. It had a 6-lug bolt pattern. You might be able to just buy the 6 lug rotors, but I'm not sure if the spindles are the same. It's worth a shot, though.
As for the rear, you could just have a machine shop drill your axles to the 6-lug pattern or get new axles.