The 4WD transfer case position sensor or the selector switch may fail causing the service 4WD message to be displayed.
Car Problem Reports
GMC Sierra 1500 HD 4WD Transfer Case Position Sensor/Selector Switch May Fail
GMC Sierra 1500 HD Problem
Drive Trains Affected: 4WD, Automatic Transmission
Average Mileage: 120,666 mi (72,000 mi - 180,000 mi)
Visitor, 2003 GMC Sierra 1500 HD, 4WD, 72,000 mi
selector switch for transfer case doesnt work and light work off and on. 4 wheel drive service light comes on. havent fixed yet will change switch