In the next pic, you can see the diff gasket ready to go, next to the diff fluid and half-built differential. The second lousy pic is a blurry rendition of the fluid inside the differential housing. And finally, the completed differential.

Not pictured is the fact that Tamiya has you put a rubber o-ring around the part of the diff cup that goes inside the diff, and then a pin through the shaft. The mid-sized gears inside the diff then sits upon that. Tamiya also provides a grub screw in the side of the differential cup, for you to drain fluid from (and insert one of the above pins easily!).

I would have snapped a pic of that, but I just thought of it. Tough nuggets.