2006 GMC Yukon Owner's Manual - Page 124

If the SERVICE 4WD message stays on, you should take your vehicle to your dealer for service. See "Service 4WD message" under DIC Warnings and Messages on page 3-56. Press and release the Four-Wheel Low button. You must wait for the four-wheel low indicator light to stop flashing and remain on before shifting your transmission in gear. Notice: Shifting the transmission into gear before the Four-Wheel Low indicator light has stopped flashing could damage the transfer case. To help avoid damaging your vehicle, always wait for the Four-Wheel Low indicator light to stop flashing before shifting the transmission into gear. It is typical for your vehicle to exhibit significant engagement noise and bump when shifting between Four-Wheel Low and Four-Wheel High ranges or from NEUTRAL with the engine running. If the Four-Wheel Low button is pressed when your vehicle is in gear and/or moving, the Four-Wheel Low indicator light will flash for 30 seconds and not complete the shift unless your vehicle is moving less than 3 mph (5 km/h) and the transmission is in NEUTRAL (N). After 30 seconds the transfer case will shift to Four-Wheel High mode. Shifting Into Four-Wheel High or AUTO 4WD (Automatic Four-Wheel Drive) Press and release the Four-Wheel High or AUTO 4WD button. This can be done at any speed, except when shifting from Four-Wheel Low, and the indicator light will flash while shifting. It will remain on when the shift is completed. Shifting Into Two-Wheel High Press and release the Two-Wheel High button. This can be done at any speed, except when shifting from Four-Wheel Low. Shifting Into Four-Wheel Low To shift to the Four-Wheel Low position, the ignition must be in RUN and the vehicle must be stopped or moving less than 3 mph (5 km/h) with the transmission in NEUTRAL (N). The preferred method for shifting into four-wheel low is to have your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h). 2-28