http://www.seattlerobotics.org/guide/servos.html

http://www.servocity.com/html/how_do_servos_work_.html

I believe that something in the control circuit was damaged by reversing the voltage.

There are three wires feeding each servo, one for powering to the servo motor, one for controlling the angle of deflection of the servo arm (proportional to the deflection of the stick in the transmitter), and one for ground, negative or current return.

Power and ground feed the motor with 4.8 or 6.0 volts.
Control and ground feed the electronic circuit and potentiometer within the servo with pulses of current
Something in those electronics and/or potentiometer was damaged by the accidental application of 4.8 or 6.0 volts.

You could try replacing either one of those parts, but it is a little more difficult than replacing the whole servo.