The purpose of using a servo is that the servo can go to many positions while a DC motor (with driver) will go from full open to full close. However, if you attach a geared DC motor on one wheel and attach a potentiometer to the other wheel (and plug it in an analog pin of your microcontroller), you can move the motor until the pot reaches a certain position then put the motor on brakes (send same signal on both direction lines, like shorting the motor leads) to hold that position. This way you will simulate a servo.