GMC Chevrolet Tahoe 2003 Manual page 128

Shifting into 4HI or AUTO 4WD
Press and release the 4 hi or AUTO 4WD switch.
This can be done at any speed (except when shifting
from 4LO), and the indicator light will flash while shifting.
It will remain illuminated when the shift is completed.
Shifting into 2HI
Press and release the 2 hi switch. This can be done at
any speed (except when shifting from 4LO).
Shifting into 4LO
To shift to the 4 lo position, the ignition must be in RUN
and the vehicle must be stopped or moving less than
3 mph (4.8 km/h) with the transmission in NEUTRAL (N).
The preferred method for shifting into 4LO is to have
your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h). Press
and release the 4 lo switch. You must wait for the
4LO indicator light to stop flashing and remain
illuminated before shifting your transmission in gear.
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Notice: Shifting the transmission into gear before
the 4LO indicator light has stopped flashing
could damage the transfer case. To help avoid
damaging your vehicle, always wait for the 4LO
indicator light to stop flashing before shifting your
transmission into gear.
It is typical for your vehicle to exhibit significant
engagment noise and bump when shifting between
4LO and 4HI ranges or from NEUTRAL with the engine
running.
If the 4 lo switch is pressed when your vehicle is in gear
and/or moving, the 4LO indicator light will flash for
30 seconds and not complete the shift unless your
vehicle is moving less than 3 mph (4.8 km/h) and the
transmission is in NEUTRAL (N). After 30 seconds
the transfer case will return to the setting last chosen.