Not a bad idea for a street car. If you can get a pair of motor kits, the batteries and electronics for 3K,.. on something like an XJ, the payoff would be at around 23k miles.(starting at 20mpg and gas at 4.00/gal) Still kind of a long wait.
Biggest problem(as mentioned above) would be environmental wear and tear on the motors, followed by handling issues due to higher unsprung weight.
It is interesting though. What would be better would be to bolt a conventional DC motor to the rear output shaft on the transfer case, and use a shortened driveshaft. Another option would be to put it between the trans and Xfr case. Any way around, you can still buy a lot of gas for 3K.
An extra 400ft/lb at the wheels would probably stop the crybabies bitching about the XJ being too slow off the line.