Good chance it's the shift fork, I've replaced mine a couple of times over the years. Worst case scenario is the gear or shaft is bad inside the trans - mine was once, it was the rear bearing on the 4wd shaft inside and required a rear split of the tractor for a bearing that cost me less than $4.00 at NAPA. The shift fork is a very simple repair. Drain the trans. fluid and remove the cover w/fork from side of trans to inspect. You can see the gear/shaft inside and check the fork for bending and/or wear. The forks aren't that expensive, I get mine from Galen at LaWall's who participates here.