I noticed a fine film of oil/grease on my rear differential just behind the last U-joint in the drive shaft. I was looking for an obvious leak but it didn't look as though my rear differential sprung any leaks. If that happened I figured there would be a lot more oil all over the place. Then I looked closer and it seemed as though it might have come from that little bolt that holds the U-joints together...(I have no idea what these parts are called). Anyway, see how the grease seems to have shot out from the top and bottom? Are these joints oil filled?