02 4.6 auto trans, manual transfercase shift. 4x4 won't always lock up. Truck off i can hear clicking noise under dash(?) and transfer case area. Could a switch be at fault or is it worse? Thanks in advance.

 

lock up? as in stay in 4wd? or what?

 

You are going to have to be a little mote specific. Basically you need to get under your truck look at the passenger side axle tube. You will see a plastic cover over your actuator. Mark the shaft and the case then get in the truck crank it up and put it in 4x4 and see if the shaft turns.