Hopefully it is just the cap or plug that is leaking. I have looked at my repair manual, and find at least two sets of part numbers, probably depending on the serial number of the tractor and whether the axle shaft is fastened with snap rings (like mine) or large nut, , but I think the cap/plug is the same. The rotating housing contains the shafts/gears that transmit the power from the differential shafts to the wheel spindle. I'd guess the bottom is open so that it can be machined and assembled. It is definitely containing the oil, as that is what lubes the shafts and bearings all the way out to the wheel hub. There is a pipe plug (5mm hex wrench) at the bottom inside of that housing for draining the oil from that lower reservoir. While your at it, change all the oil, which means the two bottom plugs, and the one in the middle under the differential. Loosening the filler plug to allow the pressure to escape is a good idea, but a bit messy, and so I made it into a breather cap, and bought a spare to check the level. I had seen some other threads suggesting that the pressure build up as the oil warms up could be causing the seals to leak.
