The front axle holder pivots on the pass through for the pto. That part is $153. There is a grease zerk on the pivot that gets missed. Either the pivot pin/pto holder housing, or the actual axle holder gets worn. There are also shims behind it for front to back movement. Many times it is the little ball joints causing most of the play. If you don't want to pay $45 for each of the little guys(#90). I usually take a mallet and smack the ends of the aluminum, right next to the ball socket. It tightens up the joint. I would inject some grease first with a greasing needle.
I will have to look deeper. It does something every tractor I have ever owned does (1961 Allis Chalmers B10, earl 2000's Simplicity Sovereign, Honda 4514) and that is when you turn the steering wheel the front axle "pivots" (moves in the direction you steer) slightly. On the Kubota it appears to be about 10-12mm. Certainly no more than I have seen it on my other machines. With the power steering it just makes it feel like it's "wandering" a tad more than the other machines did with manual steering. Perhaps it's just the "feel"? Since I never had a unit with PS. When I look at my "lines" (stripes) they're very straight. Next time I have the deck off though I will certainly look into this