Before you go to the trouble of getting it fixed, first disconnect the vent line at the diff. and blow air through the line, then disconnect the fill plug and blow air through the little metal tube attachment point, back into the diff., then change the diff. fluid. I like Amsoil Gear Lube.
When I first got my Wolverine it had a slight seep coming out of the drive shaft at the front of the diff. Changed to Amsoil and it quit for 1,600 miles. Just recently with about 1,700 miles since I had put the Amsoil Gear Lube in it, it started dripping slightly out of the rubber dust cap around the drive shaft next to the front of the diff. This time it was a good bit more than the original seep I had. I just recently changed the diff. oil again and blew air through both directions as I mentioned above. So far riding it around the yard a good bit it, it has not dripped anymore. I won't know if this stopped it for sure, until a couple of weeks from now when I plan to go ride for 3 days.
That rear diff. gets so hot that it could be possible that the heat is making the gear lube loose some of it viscosity over time and lets it leak easier. Gear Lube is designed to work in some pretty extreme hot temperatures, but I have to believe it would affect it's normal life span. With more aggressive and off camber riding it would be easy to get gear oil splashed up into the vent line and it would not take much at all to puddle in the low spot of the vent line and the pressure required to push it up and out the vent tube could be more pressure than is needed to push the gear oil pass the seal on the diff.'s front drive shaft when the oil is at it's hottest and thinnest. Not saying this is your cause for the leak, but you did have it laid over on it's side and some pretty steep grades. $15 for 1 qt. of Amsoil Synthetic 80w-90 Gear Lube is cheap compared to taking everything apart to replace the front seal. 1 qt. will do both the front and rear diffs.
This last time I was curious to see if the number of ozs. the Service Manual states to use in both the front (6.08 ozs. = 0.19 US qt.) and rear diffs. (21.76 ozs. = 0.68 US qt.) was the same as what it actually took to fill it to the brim of the filler screw, so I carefully measured the exact number of ozs. I put in, both the front and rear diff. The front was spot on taking 6 ozs., but the rear took 23 ozs. which is just a little under what the Service Manual states is required after being disassembled, but more than the 21.76 ozs. stated for just a change.
As I have discussed in another thread, I just got through replacing a rear axle and when you remove the rear axle you loose about 4 ozs. of gear oil.
Edit: After thinking more about blowing air through the vent line, in addition to this it would be even better to blow air through it and remove the line and lay it overnight in such a way where any oil that would not blow out due to it being cold and thicker would flow out the end of the vent tube. It would be impossible to completely clean 100% of the oil out of the line just by blowing air through it and it would not take much at all (overtime) draining back down from both ends of the vent tube, to the low spot on the frame, and block the tiny I.D. of the vent tube, thus still making the diff. seal leak.