[SteveO]

I think the servo will work for you. I can't say if it is the best choice because I don't know the specifications of the servo you are using and the size of your robot. Instead of using the modified servos you could use DC motors, which I think for your purposes will be pretty much the same thing.

The drawback to your design is that turning will be difficult, but if your motors are powerful enough it should not be too big of a problem. There may be an issue depending on your power supply, if you have enough for the motors. Other than that, I'm fairly sure you will be able to accomplish what you have in mind using your current setup.