The gear diff. There's only one for all their TC cars, 38 tooth pulley. Look at the listings under T4 (or T3, etc), not their gas cars or buggies, etc.Don't remember the number, but it's only one.

Beware, you will have to use Xray driveshafts with it as well because the outdrives have a larger hole than the Tamiya outdrives, so the dogbone off another car will wobble around inside.