I have a 2001 Chevy 4X4. The "Service 4WD" light comes on ocassionally. Usually goes off the next time I stop and then start up again. When the light is on, I can't switch from 2WD to 4WD. Would be a problem if it happens when I'm trying to plow. The dealer says that it is most likely a faulty sensor. He says to bring it in when the light is on. My problem, it happens maybe once a week and never when I'm near the dealership or when it's open. How do I get this fixed? Anyone else had this problem?