These are a great choice, and great price as well.
Kyosho ST-RR Hardened Differential:
http://www.ebay.com/itm/Kyosho-ST-RR...EAAOSwnipWU5zi
Here's the hardened pinions that go along with these specific diffs:
http://www.ebay.com/itm/Kyosho-1-8-I...EAAOSwZVhWRFzB
Using Mike's custom Revo shaft coupler from the store here, you can get them in 8mm bore and use Revo shafts as center driveshafts relatively easy.