I hate to say it, but I think your only recourse is to get into the transmission. Which means dropping the tracks, removing the final drives, removing the seat and laying the tranny on the bench. I would guess maybe a broken shift fork? The tranny itself is a fairly standard layout and not difficult to work in, but getting to the darn thing is a real booger! If you need a shop or parts manual, the Floyd County museum has em at a very reasonable cost.