When you pull the shift cover off you will see a roll pin holding shift arm to the shaft it is either held in with a cir-clip or a wire running through the center of it. That roll pin is broken. Kubota makes a solid pin for this, or you line the holes up, open the hole up with a 1/4" drill bit, get a piece of drill rod and make your own pin and cut your own snap-ring groove in the arm and put it back together. Or for the quick repair drive out to out roll pin and drive in a new 6mm roll pin and know that you'll be doing it again at some point.
Brian