Have a 2000 f350 superduty i have owned for short time now and 4x4 worked great when got it and now it doesnt engage. once in while i can hear it clicking when i try to to engage but light wont come on. Any ideas. I've looked underneath and wires r hooked up to a round motor lookin thing on bottom of transfer case thanks

 

I'd say you need to bench test the transfer case shift motor.