GMC 2001 Sierra 1500 Pickup Owner's Manual page 124

Shifting from 2HI to 4HI
Press and release the 4HI switch. This can be done at
any speed, and the front axle will lock automatically
with some delay.
Shifting from 4HI to 2HI
Press and release the 2HI switch. This can be done at
any speed, and the front axle will unlock automatically
with some delay.
Shifting from 2HI or 4HI to 4LO
To shift from 2HI or 4HI to 4LO, 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 4LO switch.
You must wait for the 4LO indicator light to stop flashing
and remain illuminated before shifting your transmission
into gear.
If the 4LO 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 slower than 3 mph (4.8 km/h) and the
transmission is in NEUTRAL (N). The transfer case will
shift to 4HI while waiting to complete the shift to 4LO if
you are shifting from 2HI.
Shifting from 4LO to 4HI or 2HI
To shift from 4LO to 4HI or 2HI, your 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 out of 4LO is to have your vehicle moving
1 to 2 mph (1.6 to 3.2 km/h). Press and release the
4HI switch. You must wait for the 4HI indicator light to
stop flashing and remain illuminated before shifting
your transmission into gear.
If the 4HI switch is pressed when your vehicle is in gear
and/or moving, the 4HI indicator light will flash for
30 seconds but will not complete the shift unless the
vehicle is moving slower than 3 mph (4.8 km/h) and the
transmission is in NEUTRAL (N).
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