Take out the front dog bones, and center front dog bone. Voila - 2 wheel drive.

You need to think about the fact that you will also only be able to brake using the rear wheels, so you'll be easily thrown into a spin when you hit the brakes.

I see no reason to think that going to 2 wheel drive will increase your top speed. The only advantage would be a slight reduction in drivetrain losses, which I can't imagine will result in any difference in top speed.

If you want to increase your top speed, change your gearing.