hi bruce,
i've seen this happen before on a GHEL.
The brake units on the inside of the axle might have a leak filling it up with fluid, that is forcing the fluid out. It sounds like it may have had that issue and someone just plugged off the vent and drained out some oil and let it go. Then after a while it built up more pressure from hydraulic oil coming in and blew out the next weakest point being the axle seal.
Drain out the fluid in the differential and see if it is contaminated with hydraulic oil and may have more than it should.
If this is the case that axle assembly will have to be dropped out and that piston assembly rebuilt or replaced.
Note: if you haven't worked on these before that brake assembly inside the differential has a huge spring in it that can kill you if you try to disassemble it without the proper tools. There are 2 of them, a right and left.
It's a pretty big job and i would not recommend doing it out in the field.