I just dealt with this in my truck. The switch in the CAS housing connects the light to ground. There is a small harness that goes from the switch to a connector up front on the frame. If the wire in that harness is pinched (open) the light won't work at all or if the wire is bare and touching the frame (shorted) the light will be on all the time.

Disconnect the harness at both ends and see if that makes the light go out. If it does then you know that problem is either that harness or the switch conducting to ground. If the problem doesn't go away then the issue is further up the wiring harness or the gauge cluster itself.

On my 2000 the light is an LED that is soldered to the circuit board and is not repairable nor is the circuitry that powers the LED UNLESS you have some experience troubleshooting and repairing circuit boards.

 

Possible short in the wiring.

 

The reason I don't think that it is a short is because when I dissconect the switch from the CAD, the light goes off. If it was a short wouldn't it remain on all of the time? And that would be some coincedence that it just happened to short when I used the 4wd for the first time in 12 months.
Does anyone think I should just order a new switch and see what that does?

 

If the switch does what it is supposed to when you have it in your hand, might be something else doing on in the cad. Grab a buddy, stick him in the truck. Pull the switch out of the cad, and stick a screwdriver in there, and hold it lightly against the end of the rod that the shift fork is on. (straight in....) have your buddy shift into/out of 4wd, and see how much movement you get.

Also, might wanna push/pull/tug/twist/otherwise spindle the wires with the switch out, and see if the light does anything.