You can rule out the shifter adjustment by disconnecting all the shifter rods from the trans. Make sure 1-2 and 3-4 are definitely in neutral - work the levers bolted to the side of the case til they are in neutral. Then try to manually put the trans into reverse using the bolted on lever. If it doesn't go try spinning the trans output shaft a little. If it goes into reverse you know you have a shifter adjustment problem, if not, then it's inside the trans.