As I understand it, the service 4wd light is analagous to the service engine soon light. It basically means that the 4wd computer has detected some fault. The 1st diagnostic step would be to hook it up to an appropriate scanner and "ask" the 4wd computer what fault it has seen and go from there.
I googled it the other day and alot of people are saying that it usually is the
4WD selector switch on the dash and its only about $60 from the dealership. If you want to go off you can try disconnecting the battery for about 20-30 min. and it should reset it.
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