GMC 1999 Sierra 1500 Pickup Owner's Manual page 209

Here are some things you must not do if you stall, or are about to stall,
when going up a hill.
Never attempt to prevent a stall by shifting into N (Neutral) (or
depressing the clutch, if you have a manual transmission) to "rev-up" the
engine and regain forward momentum. This won't work. Your vehicle will
roll backwards very quickly and you could go out of control.
Instead, apply the regular brake to stop the vehicle. Then apply the parking
brake. Shift into reverse, release the parking brake, and slowly back down.
Never attempt to turn around if you are about to stall when going up a
hill. If the hill is steep enough to stall your vehicle, it's steep enough to
cause you. to roll over if you turn around. If you can't make it up the
hill, you must back down the hill.
Q:
Suppose, after stalling, I try to back down the hill and decide I just
can't do it. What should I do?
A: Set the parking brake, put your transmission in P (Park) (or the manual
transmission in first gear), and turn off the engine. Leave the vehicle and go
get some help. Exit on the uphill side and stay clear of the path the vehicle
would take if it rolled downhill. Do not shift the transfer case to N (Neutral)
when you leave the vehicle. Leave it in some gear.
Shifting the transfer case to N (Neutral) can cause your vehicle to
roll even if the transmission is in P (Park) (or, if you have the
manual transmission, even if you're in gear). This is because the N
(Neutral) position on the transfer case overrides the transmission. If
you are going to leave your vehicle, set the parking brake and shi"
the transmission to P (Park) (or, put your manual transmission in f
gear). But do not shift the transfer case to the N (Neutral) positior
Leave the transfer case in the 2 Wheel, 4 High or 4 Low position.
1
Lriving Downhill
When off-roading takes you downhill, you'll want to consider a number of
things:
How steep is the downhill? Will I be able to maintain vehicle control?
What's the surface like? Smooth? Rough? Slippery? Hard-packed dirt?
Grave
I?
Are there hidden surface obstacles? Ruts? Logs? Boulders?
What's at the bottom of the hill? Is there a hidden creek bank or even a
river bottom with large rocks?
4-31