Just thought I'd tell you what I did in my '03 RAM Laramie when my Service 4x4 light came on (and stayed on). My first thought, after confirming that the wiring looked okay, was that is was my transfer case motor, I swapped in a reman motor with no effect so I replaced it with a new motor, again no improvement so I swapped in a used module (TCCM), again no change so I replaced the switch, success! Since the switch was the easiest to replace and the least expensive, at least for a used one, my recommendation is to start there. But do check your wiring first.