rear axle vent

The rear body rail on my xj is always wet looking and a few spots always seem to have gearoil on them right near where the stock axle vent on the d35 enters the body. Everytime you walk to the back of the jeep your greeted with that nice gear oil smell.

Any ideas why???

My gear oil is fresh and fill to the appropiate level. It seeps out of that dinky rubber fill plug every now and then as my shackles have raised the pinion up a little bit. I took off the rubber vent line and blew it out with air and made sure it wasnt kinked when I reinstalled it. I am curious to know why the rear diff is building up so much pressure.

Mine does this too, not positive why though. I see it as a good thing, rust proofing!!

I've been planning on running a hose from the rear axle vent all the way up to the front vent system, because I'm not sure about the condition of the 'water proof' valve cap thing back there.