GMC 2003 C4C042 Owner's Manual page 107

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The two speed axle should be in low when you park the
vehicle. You'll need to engage the transmission, even
move the vehicle a little, to be sure the axle is in the low
range. Then apply the parking brake, shift into
NEUTRAL and shut off the engine.
Also, always start the vehicle in motion with the
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two speed axle in the low range. To shift up from low to
high, do the following:
1. Move the switch up for the high range,
2. disengage the clutch, and
3. after a brief pause, engage the clutch and then press
down on the accelerator.
To shift down from high to low, do the following:
1. Move the switch down to low,
2. release the accelerator and then quickly apply
it again.
2-32
Automatic Transmission
Press the bottom of the switch to get to the low range.
The switch indicator light will come on when the
vehicle is in low range. Press the top of the switch to
return to the high range.
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The two speed axle should be in low range when you
park the vehicle. Apply the parking brake, shift to
PARK (P) or NEUTRAL (N) and shut off the engine.
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To operate off road or under a heavy load, shift into low
range when the vehicle is standing still and then operate
the transmission normally.
The rear axle shift control
switch for an automatic
transmission is located in
the instrument panel
switchbank.