Are you sure it is the diff. If it could be grease I think the drive shaft has some grease in it where it mates up to the Diff. Odds are that it is the diff though. If you are lucky it is leaking at the axle shaft. The seal is about $20 at the dealer and pretty easy to replace. If it is leaking at the pinion seal, I think it will require you dropping the diff, removing the carrier and replacing the seal. I'm no expert, but I think that will also require you to put a new shim in, reset bearing preload, backlash, etc. One of the other guys who are familiar with that will speak up soon though.
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2003 Liberty Sport 4X4
DIY Frankie Lift, BBS Rims, Aussie Locker Rear, DTT Front, Renegade Rack & Light Bar, Custom Front Bumper w/ Warn XD9000i, Custom Rear Bumper, JBA UCA's, Full Skids(Mopar, Kilby and Super skid), CRS Rock Rails, CB, EVIC, Nav, power dim mirror, (2) 8" subs under front seats.
http://www.cardomain.com/ride/2512777