I've come across 2 trucks that had this problem, a TCM and a Toyota. The Toyota overheated and warpped the forward clutch pack. That kept it engaged all the time. The TCM had a cracked steel plate in the forward clutch pack that came apart and lodged in the pack jamming it. Putting it in reverse and full throttle would make it try to go backwards a little. One easy test is to drain all the fluid so you have no pressure and if it still wants to go forward the clutch pack must come out.