I have a sportmaxx that I made (sort of) out of spare parts. I put an orion wasp 18 in it and I have a blast. It ramps pretty good, but like someone said, you loose a lot of your in air control. But it does get up to speed faster with 2wd. Once you get the hang of driving a loose diff'd 2wd, it's a ton of fun. I wouldn't recomend doing it with a 21 maxx just due to the fact that there will be so much power on one axle. You would be a lot more prone to tearing up your hole rear drivetrain.
I also have a 21 maxx that I recently tore apart. To much of a money pit.