[monk02]

The Transfer Case Control Module (TCCM) located against the wall under the glove compartment. I would start there because it is easier than any further diagnosis which gets very involved if your doing it yourself. What you are looking for is corrosion on the wires going into the wiring harness. If you have that then it will involve (going to GM dealer - not off the shelf stuff) getting a new wiring connector ($20), the F-connectors ($32), soldering skills, and some patience. After that, it will cost $500 for a new TCCM (or go to a junk yard and get one at a steal). It all worked for me 100% (knock on wood). This might explain the intermittent 4-Wheel disengagement. No guarantee. Sounds similar to my situation. Good luck.