There's a real good chance it's the front wheel assist (what most call 4x4) propeller shaft.
A couple things typically go wrong that cause the leak. First the seals will leak and second the collars that the seals ride on are rubber on the inside where they seal to the shaft, and the rubber shrinks and then some clearance opens up between the shaft and collar-causing a leak. Oil then runs into the bellhousing area and usually out of the little pin plug.
The fix is to replace both collars, shaft, and seals. This is considered a permanent fix. Some just replace the seals and that's only part of the fix. The tractor has to be split to gain access to these parts, thus its a good idea to consider doing it right once as opposed to half way doing it twice or more.
When removing the propeller shaft from the transmission case, sometimes the collar falls off inside the transmission case. Don't have a coronary, a long magnet fishes it out pretty easily and then you can usually put some grease on the transmission side of the collar, slide the "dry" side onto the propeller shaft, slide it onto the transmission and the grease will hold it well enough so that you can slide the propeller shaft back out. The thicker the grease the better obviously and you don't have to use the whole tube, just enough to coat the splines.
So what causes it?
A lot of times junk hydraulic fluid. It causes the inner diameter of the seal collars to shrink more than UDT or Super UDT does. Also severe heating of the transmission can cause it, which I see more on the HST's than the DT's or GST's; especially when using the HST in high range for moving dirt, gravel, pushing trees, etc.
Another possible leak is the input shaft of the transmission, which is fairly rare.
The rear main seal leaking is REALLY rare. Kubota did a good job on that part for sure. I can't say I've ever seen one leak that didn't have other problems.
