GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 142

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 transfer case N (Neutral) with the engine
running.
If the knob is turned to the Four-Wheel-Drive Low
position when the vehicle is in gear and/or moving,
the Four-Wheel-Drive Low indicator light will flash for
30 seconds and not complete the shift unless the vehicle
is moving less than 3 mph (5 km/h) and the transmission
is in N (Neutral). After 30 seconds the transfer case
will shift to Four-Wheel-Drive High mode.
Shifting Out of Four-Wheel-Drive Low
To shift from Four-Wheel-Drive Low to Four-Wheel-Drive
High, 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, or Two-Wheel-Drive High
position. You must wait for the Four-Wheel-Drive High,
or Two-Wheel-Drive High indicator light to stop flashing
and remain on before shifting the transmission into gear.
2-44
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.
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 transfer case N (Neutral) with the engine
running.
If the knob is turned to the Four-Wheel-Drive High, or
Two-Wheel-Drive High switch position when the vehicle
is in gear and/or moving, the Four-Wheel-Drive High,
or Two-Wheel-Drive High indicator light will flash
for 30 seconds but will not complete the shift unless
your vehicle is moving less than 3 mph (5 km/h) and the
transmission is in N (Neutral).