Kubota MX 5100HST. DISCOVERED TODAY the spline that connects both lift arms has one end with a chunk broken out. Lift arm is off shaft (spline) and hanging loose. Looks like I'm going to need to get it in to dealer. 380 hrs. Can't understand how this happen unless it got loose and slipped off then when moved it was in a bind. Hope nothing broke inside gearbox.
First Guess: The lift arm on the side where it is hanging loose was partly off the splines and hanging by a fraction of an inch when you used it and the force of using it broke off a piece of spline(s). Broken spline part is not very long is it ? Post a picture maybe ? If I am right, all you need to do is line it up and drive/force the arm back onto the splines as far as it will reasonably go. Chances are only a little bit at the end broke off and the rest of the splines (once it is firmly seated all the way on) will hold just fine. I can't remember what holds the lift arm onto the splines (?) Nut or cotter key or ? Chances of any internal damage are slim. Of course if you bought it new there the dealer will likely work with you dollar wise if the shaft actually needs replacement.