GMC 2000 Envoy Owner's Manual page 82

FIRST (1): This position gives you even more power
but lower fuel economy than SECOND (2). You can
use it on very steep hills, or in deep snow or mud.
If the selector lever is put in FIRST (1) while the vehicle
is moving forward, the transmission won't shift into first
gear until the vehicle is going slowly enough.
NOTICE:
If your rear wheels can't rotate, don't try to drive.
This might happen if you were stuck in very deep
sand or mud or were up against a solid object.
You could damage your transmission.
Also, if you stop when going uphill, don't hold
your vehicle there with only the accelerator pedal.
This could overheat and damage the transmission.
Use your brakes or shift into PARK (P) to hold
your vehicle in position on a hill.
2-20
Tow/Haul Selector Switch
You can use this feature to more efficiently tow or haul
a heavy load. See "Tow/Haul Mode" in the Index for
more information.
Your vehicle is equipped
with a tow/haul selector
switch. This switch is
located on the shift knob.