hear is the gearing app:
http://www.scriptasylum.com/rc_speed/index.html
It is found in the first sticky in the brushless section.
It was all written by BrianG, a moderator here. Just start clickinmg around in it. It's all drop down menus and super easy to use, and very very helpful.
That ofna spider diff should work great. I'd start with 10,000wt oil in it and adjust from there.