GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 139

Shifting into Four-Wheel-Drive Low should be done, if
possible, with the vehicle at a slight roll, 3 mph (5 km/h)
or less.
•
Shift the transmission into N (Neutral).
CAUTION:
{
Shifting the transfer case to 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 Neutral. See Parking
Brake on page 2-50.
•
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 Four-Wheel-Drive Low do not drive faster
than 45 mph. This will reduce wear and extend
the life of your transfer case.
Shifting In or Out of Neutral
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.
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