I noticed a stain on my Libby's rear axle also, where the right axle tube is pressed into the rear diff. I went to Chrysler and the tech said I needed a new rear end, and that it is warrantied (I have 20k miles on it). He said the tube is pressed in with RTV and the seal isn't working so the only thing they can do is get a new rear axle/diff, and put my ring and pinion inside... They will not switch to a powertrax or a Jeep LSD (unless I buy the LSD new from them), nor will they put in synthetic fluids.... Oh well, at least I get a new rear axle. An extended breather tube will come shortly after, because you never know why it's leaking in the first place...

The rear end is on order, appointment sometime next week.
By the way, they asked if I did the first diff service, which is a drain and refill at 12k miles (Service Schedule B) and I just told them my driving did not fit Service Schedule B and so I had not come upon a diff service yet (using schedule A)... FYI....