If you are looking for basics on setup, both the Losi and Asso manuals provide information and what changes do for you. In a nutshell setup changes ar no different from what you do with a touring car or any other vehicle. It is just that for a 2WD your emphasize might been on different changes compared to a TC or an 1:8 buggy.
You are right, that a ball and gear diff do the same thing. If you are running a ball diff on a high traction surface you might be challenged to stop if from slipping while still having the required "diff effect" A gear diff by design can't slip while the diff effect is changed by using different silicon oil.
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