GMC 2012 Sierra Series Owner's Manual page 331

If the transfer case shifter is in
the N (Neutral) position and you
have difficulty reaching the
selected transfer case mode,
with the engine running, shift the
transmission momentarily to
D (Drive) and then back to
N (Neutral). This will realign the
gear teeth in the transfer case
and allow you to complete the
shift.
Shifting from Two-Wheel Drive
High to Four-Wheel Drive High
Shifts between Two-Wheel Drive
High and Four-Wheel Drive High
can be made at any vehicle
speed.
Shift the transfer case lever in
one continuous motion into
either the Four-Wheel Drive High
or Two-Wheel Drive High
position.
Shifting from Two-Wheel Drive
High to Four-Wheel Drive High
while the vehicle is in motion
may require that moderate force
be applied to the shift lever for a
few seconds before Four-Wheel
Drive High can be engaged,
especially in cold weather.
In extremely cold weather, it may
be necessary to slow or stop the
vehicle to shift into Four-Wheel
Drive High.
While in Four-Wheel Drive High,
the vehicle can be driven at any
posted legal speed limit.
I n f o r ma t i o n P r o v i d e d b y :
Driving and Operating
Shifting In or Out of Four-Wheel
Drive Low
Notice: Shifting the transfer case
into Four-Wheel-Drive Low while
moving at speeds faster than
5 km/h (3 mph) may cause
premature wear to the transfer
case, and may cause the gears to
grind. To avoid causing
premature wear, and grinding the
gears, do not shift the transfer
case into Four-Wheel-Drive Low
while the vehicle is moving faster
than 5 km/h (3 mph).
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.
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