GMC TopKick 2007 Owner's Manual page 119

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4 (Four-Wheel Low): This setting sends
maximum power to all four wheels. You might
choose Four-Wheel Low if you are driving off-road
in deep sand, deep mud, and climbing or
descending steep hills. Be sure the front wheel
hubs are locked when you want to drive in
Four-Wheel Low.
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 injured. If you
are going to leave your vehicle, set the
parking brake and shift the transmission
to PARK (P) and make sure the transfer
case is in a drive gear.
Notice: If your vehicle has an Allison
transmission and you try to put the
transmission in PARK (P) while the transfer
case is in Four-Wheel Low, the transmission
might not go into PARK (P) and could damage
your vehicle. When parking your vehicle,
make sure the transfer case is in Two-Wheel
High or Four-Wheel High.
NEUTRAL: Shift the vehicle's transfer case to
NEUTRAL only when towing your vehicle.
Shifting Into Four-Wheel High
Turn the knob to four-wheel high. This can be
done at 3 mph or less, while you are shifting out
of Four-Wheel Low. The indicator light flashes
while shifting. It remains on after the shift is
complete. Be sure to lock your front hubs.
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