GMC 2011 Sierra Owner's Manual page 360

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 5 km/h (3 mph)
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
1.6 to 3.2 km/h (1 to 2 mph). 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.
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 5 km/h (3 mph)
and the transmission is in
N (Neutral).
To shift the transfer case to
N (Neutral) do the following:
1. Make sure the vehicle is parked
so that it will not roll.
2. Set the parking brake and
apply the regular brake pedal.
See
for more information.
3. Shifting the transfer case into
N (Neutral) can be done with
or without the engine running.
Shifting without the engine
running should be done with
the ignition in ON/RUN.
4. Put the transmission in
N (Neutral).
5. Shift the transfer case to
Two-Wheel Drive High.