Did you change the rear diff fluid yourself or have it done. If you did it yourself did you replace the crush gasket or use the old one? Mine also looks like that but it was only because when i refilled with new fluid it ran over and I wiped it off. Based on the picture i dont see to much if it is leaking.
It just looks like it was serviced… fresh fluid or a fluid top-off. Unless you clean up with something like brake cleaner, gear oil stays pretty "wet" and oily. When you get under there, wipe it down with liberal amounts of brake cleaner so you get all of the oil off and check it again in a few days. If it "wets" again, you may need a new crush washer as mentioned above… probably would be a good idea to drain and fill with fresh oil if thats the case.