GMC 2010 Sierra 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.
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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).