build a gmaxx and i need help

[suicideneil]

It is possible.
You dont need/ have to have one.
You can use the stock centre diff option traxxas cell for the erevo/3905 transmission, or you can get Mr.Crash to make you one that uses a proper 1/8 scale diff as the basis.

The centre diff is usefull if you want to race, or if you have trouble with too much power/ wheelies, in which case you can tune the CD to unload some of the power to the front wheels, meaning fewer wheelies and more time spent the correct way up.

traxxas CD

1/8 scale CD

The traxxas option is an easy drop-in unit, but it is difficult to tune well. The 1/8 route is much better, but requires you to either build your own by following the various guides in that 2nd thread above, or Mr.Crash can build you one if you dont feel confident about it.

For general bashing though, I find I can live without a CD, I just use a very long wheelie bar instead...