Chevrolet 2008 Silverado 1500 Pickup Owner's Manual page 142

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 requested mode indicator light has
stopped flashing could damage the transfer case.
To help avoid damaging your vehicle, always
wait for the mode indicator lights to stop flashing
before shifting the transmission into gear.
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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).