Nitro's make poor crawlers... spool up time with the clutch kill's low end grunt and the ability to move slow with any power. You can make a T into an OK crawler/rock basher with spools front/rear, biggest spur(72T IIRC) smallest bell(15T IIRC) tweak the clutch to engage LATE and lock it into FIRST gear for the most torque( I'd leave the tranny as is) DOWNSIDE'S are it will never be a good crawler, you'll have to keep a close eye on temps and you'll have to be extra careful jumping/bashing around with it due to the spool's... or carry lot's of diff screws, gears, half shaft's and go like hell till it brakes
the best(IMHO) out of the box nitro crawler/basher is this
http://www.hobbypeople.net/gallery/145642.asp
it's still not what I would call a "good" crawler.
John