take the rear dogbone out, and stick a screw driver in the little hole in the gear box (the thing the dogbone sticks into) (forgot which side it's on), tighten the screw all the way down then back off about 1/3 or so turns...in answer to your question no you don't need to disassemble the gearbox, and yes it is a very helpful tuning option...also make sure you have some good grease, or lubricant so your diff is smooth...

too tight- wheelies, bad cornering, will spin out easier

too loose- slower through turns but more forgiving b/c you won't spin out

One of my xtreme rc magazine articles had everytihng about the diff in it...check out rc411.com