Does it even try to go reverse if you rev it up? Sounds like you're on the right track. Is there a cover on the trans that you can remove to see inside? If so you could probably take a compessed air gun and apply air pressure to the F and R test ports and watch and listen to the clutch pack. Put the lever in gear while doing this. You should be able to hear the reverse one leaking bad if the piston is cracked or any or the seal rings are blown. That would prove you have to take it out.