Your car most likely - I'm assuming it's an RTR - came with grease in the diffs.  From the photos, that looks like very old and dried up grease.  You wanna clean all that gunk outta there with motor spray or some sort of light solvent like Simple Green or what I like to use is Goo Gone.  Then fill the diffs with diff fluid...

The outside gear, the ring gear, should have some grease on it to help lube the ring gear and the pinion gear inside the diff case.