Every time we take it out of 4WD we get this message. Sometimes if we are moving at 25 MPH and shift back to 2WD will we complete the shift without a problem. I've also heard that this can be caused by a defective transfer case switch, but I don't know how much that will be to fix, this car has done this since new.

the "service 4w drive" light comes on almost every other day. It has been taken to multiple Chevy dealers and mechanics who can't figure it out either. I didn't even use it yet and the message was popping up! Still does today, 8 years later!!!