GMC 2007 Yukon XL Owner's Manual page 149

It is typical for your vehicle to exhibit significant
engagement noise and bump when shifting
between Four-Wheel Drive Low and Four-Wheel
Drive High ranges or from NEUTRAL with the
engine running.
If the knob is turned to the Four-Wheel Low
position when your 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 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 Drive High mode.
Shifting Out of Four-Wheel Drive Low
To shift from Four-Wheel Drive Low to Four-Wheel
Drive High, AUTO 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). Turn the knob to the
Four-Wheel Drive High, AUTO or Two-Wheel
Drive High position.
You must wait for the Four-Wheel Drive High,
AUTO or Two-Wheel Drive High indicator light to
stop flashing and remain on before shifting
your transmission into 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 Drive Low and Four-Wheel
Drive High ranges or from NEUTRAL with the
engine running.
If the knob is turned to the Four-Wheel Drive High,
AUTO or Two-Wheel Drive High switch position
when your vehicle is in gear and/or moving,
the Four-Wheel Drive High, AUTO 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 NEUTRAL (N).
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