Is that the Hydroshuttle transmission? I have a L3750 which has 4 ranges on the right hand side by the seat. Then you shift forward and reverse up on the column. There are a lot more Kubota fans on here that might have other troubleshooting ideas. Reverse is working ok?
Does the PTO engage? Not that it matters much I am just checking for things that I would try. The shift fork scenario sounds plausible. I haven't had any problems with my machine like that at all.
Have you called any Kubota service centers and picked their brains?