[suicideneil]

Slipperential.

The slipperential replaces the tranny with a centerdiff and slipper clutch-in-one, so you would still have to use the standard diffs front & rear- there is simply no way you could run different teeth counts in the front vs the rear, simply impossible. As for a 'front one way' (I assume you mean forwards only driveshaft that free wheels in reverse?), that would lead to very poor braking in a 4wd vehicle.

You might be able to retro-fit a center diff unit into the MT2 to replace the tranny, and use Mike's unit in place of the center diff itself, but thats about it.

As for you BL E-revo 4s or 6s question:

4s for a fast (~40mph) reliable system that will run pretty cool with plenty of grunt.
6s for insane speed and power that is only much use in a huge open space (speed runs or speed bashing).

Gearing up or down with either system will be the key to controllability and consistent, safe temps.