Correct, most servos can't turn a full revolution. They're usually used for very precise movement back and forth of components in robots. For example, in a remote controlled plane, you would have a servo controlling the flaps, moving them up and down, or in an rc car, you would have a servo turning your steering left or right.
If you want a motor to drive a track continuously, look at using brushless DC motors if you want speed, or if you want some finer control, look at using stepper motors. Your best bet would be brushless DC. Depending on the size of the motors, our 1060, or 1064 motor controllers would work well for you. They can each control two motors, so you could buy one controller, and use it to drive your left and right motors.
Here's a tank I made that might give you some ideas:viewtopic.php?f=13&t=3142