It's possible you have a tweaked or bent axle, though with a stock-ish Jeep that hasn't been on any collisions, it's less likely. If you get an alignment, have them print out the results for the rear axle (it can't be realigned, but it will tell you if there is an issue) and compare to the factory specifications.
Or it could simply be that the seals were getting old and cracking from age and use.
Have you checked to make sure the vent line isn't plugged or that the check valve isn't malfunctioning? The end of the vent line and the check valve are located immediately behind the fuel filler. If these get clogged, the axle can get over-pressurized and force oil out random seals, even if they are otherwise normal. (If you frequently ford deep water (over the hubs), you may consider switching to a different kind of breather like the ARB, which also prevents underpressure.)
Also check the fluid level in the axle. If it gets overfilled, it can force fluid out of the seals. It may be worth changing the fluid anyways at the same time as a seal replacement.