2004 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 Driver Information Center (DIC) on page 3-56. Shifting into 4HI or AUTO 4WD Press and release the 4HI or AUTO 4WD button. This can be done at any speed (except when shifting from 4LO), and the indicator light will flash while shifting. It will remain illuminated when the shift is completed. Notice: Shifting the transmission into gear before the 4LO indicator light has stopped flashing could damage the transfer case. To help avoid damaging your vehicle, always wait for the 4LO indicator light to stop flashing before shifting the transmission into gear. It is typical for your vehicle to exhibit significant engagment noise and bump when shifting between 4LO and 4HI ranges or from NEUTRAL with the engine running. If the 4LO button is pressed when your vehicle is in gear and/or moving, the 4LO indicator light will flash for 30 seconds and not complete the shift unless your vehicle is moving less than 3 mph (4.8 km/h) and the transmission is in NEUTRAL (N). After 30 seconds the transfer case will shift to 4HI mode. Shifting into 2HI Press and release the 2HI button. This can be done at any speed (except when shifting from 4LO). Shifting into 4LO To shift to the 4LO position, the ignition must be in RUN and the vehicle must be stopped or moving less than 3 mph (4.8 km/h) with the transmission in NEUTRAL (N). The preferred method for shifting into 4LO is to have your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h). Press and release the 4LO button. You must wait for the 4LO indicator light to stop flashing and remain illuminated before shifting your transmission in gear. 2-30