e maxx diff confusion :s

[rusty rustler]

hello first time posting on here

im looking to convert my emaxx to brushless and i have herd that the diff are a must on the upgrading list, im opting for the monster mamba setup and want to get the diffs done before buying the motor and esc. Need to know what to get so if anyone has a currant setup that works or know what to get and diffrent options help would be greatly appreciated