If it was me, I'd probably get a PuddyMod rear differential. PuddyMod is a member on here that makes some excellent products. You can see the rear diff here about a quarter of the page down:

https://www.s2ki.com/forums/index.php?showtopic=558914

You should also be able to order it with a longer gear ratio. People have also adapted Ford rear diffs, and Inline Pro makes a kit as well (but I believe it requires new axles) that is extremely strong but also very $$.

I'd check with the FI guys and see what their consensus is with rear diff upgrades. I'm sure there is a general list of "most likely to break" parts, so after the diff may be your axles, and after your axles...etc.

 

it has a 1.5 way diff stock bro

 

Wow, that guy is awesome!!! im defiantly going to get a few of his things!! ill make sure to mention your name as a reference, maybe he will throw you a bone!!!