That's the axle seal and the gear lube is coming out of the tube onto the knuckle. There will be a seal inside the housing that you will have to get to. You'll need to remove the carrier to get access to the seal. While you're in there I would just do both sides (passenger seal is on the outside of the disconnect housing, will need a slide hammer and a friend for this side). Depending on the wheeling you do, now would be a good time to think about moving to a TJ non-disconnect setup and installing a pair of axle tube seals to keep all the crud out of the seals. I ate the passenger side seals up 3 times before doing a disconnect-eliminator kit and tube seals. Haven't had a problem since.
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