Farm Fourtrax Transmission Woes

Hey folks, been using this site for research for awhile but I may have inherited our old farm Fourtrax so I figured I join up and pick yall’s brains.

We’ve got an old Fourtrax (95 I believe) that’s been on the farm for the past twenty years. Three of us have grown up with this machine so it has some sentimental value to me. It’s always been tried and true but lately the issues have been stacking up and my uncle was ready to give up on the old girl. I stepped in and said I wanted a crack at it before she heads to Craigslist.

Our main issue is that the transmission seems to be locked up. It cranks and runs fine. When you put it into gear it will "bow up" like it wants to roll, but won’t budge. Keep in mind this is a farm machine that has been ridden hard and put up wet. In all likely hood it has not gotten the maintenance it deserves. Where should I start to diagnose this problem? Thanks for any help!