You can use the encoder you have on the elevator motor. The proportional controller that Greg is referring to can be implemented easily in any language. If you use Labview, it is available to you on the vi pallette. If you are using C++ or Java, it is easy to implement. The PIDController Class is described well here:
http://www.wbrobotics.com/javadoc/ed...ontroller.html
All any of the code does is compare (subtract) your current encoder value from the desired encoder value, giving you an error value. The (P)roportional controller multiplies the P constant times the error and scales your motor output. As the error gets less, you can see that the motor value will fall, slowing down the motor.