GMC 2006 Sierra 1500 Pickup Owner's Manual page 138

Shifting into Four-Wheel High or AUTO
4WD (Automatic Four-Wheel Drive)
Press and release the four-wheel high or AUTO 4WD
switch. This can be done at any speed, except when
shifting from Four-Wheel Low, and the indicator light
will flash while shifting. It will remain on when the shift is
completed.
Shifting into Two-Wheel High
Press and release the Two-Wheel High switch.
This can be done at any speed, except when shifting
from Four-Wheel Low.
Shifting into Four-Wheel Low
Notice: Shifting the transfer case into
Four-Wheel Low while moving at speeds faster
than 3 mph (5 km/h) 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 Low while the vehicle is moving
faster than 3 mph (5 km/h).
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To shift to Four-Wheel Low, the ignition must be in RUN
and the vehicle must be stopped or moving less than
3 mph (5 km/h) with the transmission in NEUTRAL (N).
The preferred method for shifting into Four-Wheel Low is
to have your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h).
Press and release the Four-Wheel Low switch. You must
wait for the Four-Wheel Low indicator light to stop
flashing and remain on before shifting your transmission
in gear.
Notice: Shifting the transmission into gear before
the Four-Wheel Low indicator light has stopped
flashing could damage the transfer case. To help
avoid damaging your vehicle, always wait for the
Four-Wheel Low indicator light to stop flashing
before shifting the transmission into gear.
It is typical for your vehicle to exhibit significant
engagement noise and bump when shifting between
Four-Wheel Low and Four-Wheel High ranges or from
NEUTRAL (N) with the engine running.
If the Four-Wheel Low switch is pressed when your
vehicle is in gear and/or moving, the Four-Wheel Low
indicator light will flash for 30 seconds and not complete
the shift unless your vehicle is moving less than
3 mph (5 km/h) and the transmission is in NEUTRAL (N).
After 30 seconds the transfer case will shift to four-wheel
high mode.