Have you tried disconnecting the cable and moving the selector on the gear box by hand. I had a similar problem once where the mechanism on the in the shifter had came loose and wasn't moving the cable in full motion in either direction. Thought it was the trans at first until I disconnect the cable and was able to shift the trams manually. That in turn led me back to the shifter which when removed was easy to see and repair the problem.
I always try to start with the simple stuff first and work my way back from there.