GMC Sierra 2008 Owner's Manual page 135

CAUTION:
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Shifting the transfer case to NEUTRAL
can cause your vehicle to roll even if the
transmission is in PARK (P). 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 2-48.
N (NEUTRAL): Shift the vehicle's transfer case
to NEUTRAL only when towing your vehicle.
See Recreational Vehicle Towing on page 4-50 or
Towing Your Vehicle on page 4-49 for more information.
If the SERVICE 4–Wheel Drive message stays on,
you should take your vehicle to your dealer for
service. See "Service 4–Wheel Drive message" under
DIC Warnings and Messages on page 3-67.
Shifting Into Four-Wheel-Drive High or
AUTO (Automatic Four-Wheel Drive)
Turn the knob to the Four-Wheel-Drive High or AUTO
position. This can be done at any speed, except
when shifting from Four-Wheel-Drive Low, and the
indicator light will flash while shifting. It will remain
on when the shift is completed.
Shifting Into Two-Wheel-Drive High
Turn the knob to the Two-Wheel-Drive High position.
This can be done at any speed, except when shifting
from Four-Wheel-Drive Low.
Shifting Into Four-Wheel-Drive Low
When Four-Wheel-Drive Low is engaged, vehicle speed
should be kept below 45 mph. Extended high-speed
operation in Four-Wheel-Drive Low may damage
or shorten the life of the drivetrain.
2-45