Is it one of the rear axle seals? If so, the crappy vent on the axle is what a lot suspect as the culprit. The axle vent is not continuous, just a check valve so once the axle cools you will basically be pulling a vacuum on your axle seals, which can cause premature wear. Do a search for 'axle vent mod', which a few on here have done. This involves replacing the check valve with a continuous vent so the axle can breathe. That seems to be a cheap and effective solution for those with axle seal issues.