2010 GMC Sierra 1500 Crew Cab Owner's Manual - Page 190

Shifting Out of Four-Wheel-Drive Low To shift from Four-Wheel-Drive Low to Four-Wheel-Drive High, AUTO or Two-Wheel-Drive High, the vehicle must be stopped or moving less than 3 mph (5 km/h) with the transmission in N (Neutral) and the ignition in ON/RUN. The preferred method for shifting out of Four-Wheel-Drive Low is to have the vehicle moving 1 to 2 mph (1.6 to 3.2 km/h). Turn the knob to the Four-Wheel-Drive High, AUTO or Two-Wheel-Drive High position. You must wait for the Four-Wheel-Drive High, AUTO or Two-Wheel-Drive High indicator light to stop flashing and remain on before shifting the transmission into gear. Notice: Shifting the transmission into gear before the requested mode indicator light has stopped flashing could damage the transfer case. To help avoid damaging the vehicle, always wait for the mode indicator lights to stop flashing before shifting the transmission into gear. It is typical for the vehicle to exhibit significant engagement noise and bump when shifting between Four-Wheel-Drive Low and Four-Wheel-Drive High ranges or from N (Neutral) with the engine running. If the knob is turned to the Four-Wheel-Drive High, AUTO, or Two-Wheel-Drive High switch position when the vehicle is in gear and/or moving, the Four-Wheel-Drive High, AUTO or Two-Wheel-Drive High indicator light will flash for 30 seconds but will not complete the shift unless the vehicle is moving less than 3 mph (5 km/h) and the transmission is in N (Neutral). 3-52