Many years ago when I was a kid I tried to make my Tamiya Blackfoot 4WD with parts from a Tamiya Hot Shot. I failed. I got the rear gearbox mounted but the front was outside my range of abilities. From my observations the front of the chassis could not support a front drivetrain. Little did I know that it was possible. Take a look at this article from a Japanese magazine. It shows that with the addition of Tamiya TA04 parts you can convert a Tamiya Frog (same chassis as the Blackfoot) to 4WD. I know this car won't win any races but the awesome factor is through the roof. Check it out.
UPDATE: There's a 4WD Hornet too!
UPDATE: There's a 4WD Hornet too!