I have been having the same problem. I just had the dealer install a "4th" left rear axle seal in 8 months and finally a new left rear axle shaft (under extended warranty). Hopefully this works but I'm not holding my breath. From my research, besides a poor job installing the seal, any burs, or warpage of the axle shaft, or the axle housing can cause repeated axle seal failures. Also check to be sure your breather tube is clear of any junk. It runs from the top left of the rear differential and parallels the gas fill spout. If this tube is clogged then the gear oil has to go somewhere under pressure and commonly spits right out the ends of the axle. Good luck finding your problem!
2000 TJ Sport 4.0L
31' Kumho Venture M/T, V-Force Muffler, Hi-lift, Midland CB, Hella 500 Fogs, Tuffy Center Console, Hurst T-handle shifter, X-Bound Seat Covers