You don't want a "Diff" on a crawler (Front. rear OR center). The problem a diff will cause, is when a wheel or axle gets stuck (Which will happen while crawling), it will lose all power to that axle, and just spin the other wheels. You want to run a locked front and rear axle, with a locked tranny. This way, all 4 tires are always getting power.

Look into a Scorpion tranny, it has a VERY big gear reduction, which will send a lot more pulling power to the wheels. Also, look at a solid axle setup over independent suspension. both will work, but the articulation of an independent suspension, will keep the tires parralel longer, not letting the "working" part of the tire do it's job. Crawler tires get the best bite from the surface of the tire, very little usable bite from the sidewall.