Re: Rear Differential Problems!!!
2 Like causes are:
1: Gasket/RTV seal between the case and the housing is leaking. If it looks like it is wet between the seams, this is probably the cause. Simple to fix yourself and it wouldn't cost much to have fixed at a shop. The biggest cost will be the 75W140 gear oil (about $50). If you go off road you could have also hit the diff off some rocks and bent it a little. Also, not a big deal to fix.
2: Rear pinion seal leaking at the propeller shaft. If it looks like it is wet higher up, where the drive shaft enters and is leaking down - this is the likely culprit. Not the easiest job to do yourself (requires special tools and know-how to get the bearing pre-load correct). Looking at around a $200 bill to get this taken care of at a shop.
Depending on how badly it is leaking (gear oil always looks like it is a bigger leak than it is), you'll want to keep an eye on the fluid level until you get it taken care of.