[GO-RIDE.com]

As others have said there is no need for a rear, and I believe you would only use the front or the center, but not both together. My understanding is that if you use the front smart diff you get the most amount of 2wd feel as the front wheels will now be seperate from each other when off throttle. If you use the center smart diff you get a sort of in between 2wd and 4wd feel as there is still some diff drag between the left and right front wheel.

I think the problem for most "e" conversions will be that if you use a smart diff you will only have rear wheel braking.