For just replacing what u have you probably stirpped the gears in the diff.
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&W=000646836&I=HPIC7855&P=K]Final Gears[/link]
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXTB43&P=X]Diff Case[/link]
You wont have to have the diff case. I bought one so i could get the shims that come with it. If yours is like mine you will find no shims installed from the factory, and the gears will be stripped or rounded off only on the tips. The gear mesh is to loose and the gears wear out prematurly.

Hpi is comming out with some heavy duity gears if you want to up grade to these you will need the following parts.
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&W=000646836&I=LXJYM2&P=K]Heavy duity gears[/link]
[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&W=000646836&I=LXDLG8&P=K]Pins to hold the drive cup to the gear[/link]
The drive cup i cannot find yet, but the HPI part number for that is 86341.

Here are the gears on HPI's web page
[link]http://www.hpiracing.com/index2.htm[/link]

HPI's part number for the gears is 86336

HPI's 86329 gears look like they would drop righ in. They look the same as the factory replacement parts. It says that they are taller gear ratio though. Different gearing option if you were running a stout engine that could turn the gears.