4 Wheel Drive/SUVs: automatic 4 wheel drive, wheel drive, wheel

Question
I own a 2005 Sierra 1500 4x4 pick up truck and my automatic 4 wheel drive works occasionally. The lights on 2hi and 4hi do not come on. My computer also reads service 4 wheel drive occasionally! Is this a sensor or is it a more serious problem?

Answer
Vincent, it is not a sensor problem , more than likely the problem is either the transfercase motor , or might be in the transfercase itself without checking the motor there is no way to be sure , thanks and let me know if you need instrutions on how to check the motor ADAM