GMC Sierra Classic 2007 Owner's Manual page 160

Shifting Out of Four-Wheel Drive Low
To shift from Four-Wheel Drive Low to Four-Wheel
Drive High, AUTO 4WD, or Two-Wheel Drive
High, your vehicle must be stopped or moving less
than 3 mph (5 km/h) with the transmission in
NEUTRAL (N) and the ignition in RUN. The
preferred method for shifting out of Four-Wheel
Drive Low is to have your vehicle moving 1 to
2 mph (1.6 to 3.2 km/h). Press and release the
Four-Wheel Drive High, AUTO 4WD, or
Two-Wheel Drive High switch. You must wait for
the Four-Wheel Drive High, AUTO 4WD, or
Two-Wheel Drive High indicator light to stop
flashing and stay on before shifting the
transmission into gear.
Notice: Shifting the transmission into gear
before the Four-Wheel Drive Low indicator light
has stopped flashing could damage the
transfer case. To help avoid damaging your
vehicle, always wait for the Four-Wheel
Drive Low indicator light to stop flashing
before shifting the transmission into gear.
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The transfer case may make noise and the vehicle
may jerk when the transfer case is shifted
between Four-Wheel Drive Low and Four-Wheel
Drive High ranges or from NEUTRAL with the
engine running.
If the Four-Wheel Drive high, AUTO 4WD, or
Two-Wheel Drive High switch is pressed when
your vehicle is in gear and/or moving, the
Four-Wheel Drive High, AUTO 4WD, or
Two-Wheel Drive High indicator light will flash for
30 seconds but the vehicle will not complete
the shift unless it is moving less than 3 mph
(5 km/h) and the transmission is in NEUTRAL (N).