2000 silverado service 4wd comes on within 30 secs of selecting any 4wd mode. Shifts into the selected mode. Disables the ability to shift out of the selected 4wd mode until; stop the truck, put in park, turn ignition off, remove key, open dr side door, close the door, put key back in, turn ignition on, press 2WD button. Then Service 4WD is cleared and transfer case shifts back to 2WD. What is wrong? 4WD control switch is new GM.

4WD Auto functions - service 4wd on at 30 seconds
4WD functions - service 4wd on at 30 seconds
4WD low functions - service 4wd on at 30 seconds