I have nothing to add to the outrunner side of things sorry. But just to clarify for jhautz, crawlers run no true 'diffs'. I.E if you move one tire every other tire moves exactly the same amount. The overdrive is used to help torque twist, steering and having the front pull a little keeps the wheels on the ground in steep climbs. At such low speeds in crawlers you don't notice the tire scrub side of things anyway.

Cheers
Kieren