you can still pay the 3 bucks for a replacement differential, or the 15 bucks for the aluminum one. I personally have gone w/ the aluminum one. but if you adjust the slipper correctly you can save a few bucks and still use the "stock" differntial the cheaper one. But you will have to make sure its adjusted correctly, and the slipper is slipping before the differential.

That part number is from Robinsonracing.com
you can get the part cheaper from ebay or your LHS possibly. The rings are available from horizonhobby.com or someplace that sells losi stuff. I am sure you can the the robinson racing differential gear there.