I have had this problem for quite a while now. A year maybe? Unfortunately I havent had any extra money to maintain my truck like it should be...Anyway, on the inside of the rear driver side wheel there is a buildup of thick black grease. It also appears to be coating the caliper as well as other parts around the wheel and brake. I thought maybe the differential might be leaking or something, but I looked and there is little to no evidence of leakage on the differential itself. I know this is not just a grime buildup from the road, because this is the only wheel that it is on. Any ideas what this could be?

 

Well, really the only grease on the rear wheels, are the differential fluid that's inside the diff its self. Check your differential fluid level, and add fluid if necessary.

It's likely that you need an axle seal.