GMC 2011 Sierra Owner's Manual page 352

Shifting into Four-Wheel Drive
Low should be done, if possible,
with the vehicle at a slight roll,
5 km/h (3 mph) or less.
Shift the transmission into
N (Neutral).
Shifting the transfer case to
N (Neutral) can cause the vehicle
to roll even if the transmission
is in P (Park). You or someone
else could be seriously injured.
Be sure to set the parking brake
before placing the transfer case in
N (Neutral). See
.
Shifting into Four-Wheel Drive
Low with the vehicle at a stop
may be more difficult. You may
be unable to complete the shift
to Four-Wheel Drive Low, and
the transfer case will end up in
N (Neutral). This is normal, and
is a function of the gear teeth
aligning in the transfer case.
When this happens, make
sure the engine is on, shift
the transmission momentarily
to D (Drive) and back to
N (Neutral), and then complete
the transfer case shift.
Shift the transfer case shift lever
in one continuous motion into
the Four-Wheel Drive Low
position.
When in Four-Wheel Drive
Low do not drive faster than
72 km/h (45 mph). This will
reduce wear and extend the
life of your transfer case.
1. With the vehicle running and the
engine at an idle, set the parking
brake.
2. Place the transmission into
N (Neutral).
Shift the transfer case in one
continuous motion into or out of the
N (Neutral) position.