Thats why i leave it in the ON position which is actually Off.
Any ideas on what it might be or should i just check the wiring?
The vacuum lines to the actuator are reversed if its really disengaged in the on position.
Pop the hood, look for the two solenoids that control the diff lock. Two vacuum hoses come from the manifold, and two hoses go back to the transmission. Switch the two going to the transmission and it should work fine.
With the switch in the ON position, does it bind around corners? Try turning it in tight circles, then flip the switch to OFF and try it real quick. You'll notice right away which one is and isn't in the engaged position. (If it gets stuck, just drive in reverse in the same tight circle to unbind it, or lift one end of the car up in the air)