Well, there's supposed to be a vent tube to allow the axle to breathe as temperatures change... maybe it fell off? It would be a black rubber or plastic line about 2 feet long connected to the vent fitting and just tie-wrapped to the floor of the trunk. I don't remember exactly what it's tied to but it may have just gotten disconnected. MAybe it's been off for a while and it spit out a little gear oil from it. Reattach it, use a hose clamp or tie wrap to keep it on there, and clean up with Brakleen or similar degreaser- drive and look for leaks afterward.
Usually the only problem with differential leaks is the pinion seal where the driveshaft connects. If that's OK, I wouldn't worry much about the axle. Do check the fluid level to make sure it didn't lose much oil from that hose being off. These axles are Dana 30's - same model as a 4WD Jeep front axle. They're pretty darn tough and reliable.
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Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"