Originally Posted by Thunderhorse
4wd auto is not the same as AWD. AWD can vary power to each of the 4 wheels, usually through a center differential. 4wd auto runs your truck in 2wd until the rear slips, and the front wheels kick in. It works in a sort of similar manner to AWD, but cannot vary power to all of the wheels; it can only vary power to each wheel per axle since there is a differential for the front and one in the rear. Both driveshafts are locked via the transfer case, so it can vary power between the front right and front left wheels for example, but unlike AWD it cannot vary power between the front left and rear right wheels.
That's why AWD can be driven on dry paved roads, but 4wd can't unless its in auto in which case you will really be in 2wd to prevent binding the transfer case.
From the 2013 Ram 1500 owner's manual:
Electronically Shifted Transfer Case
(Five-Position Switch) – If Equipped
This is an electronic shift transfer case and is operated by
the 4WD Control Switch (Transfer Case Switch), which is
located on the instrument panel.
This electronically shifted transfer case provides five
• Two-wheel drive high range (2WD)
• Four-wheel drive automatic range (4WD AUTO)
• Four-wheel drive lock range (4WD LOCK)
• Four-wheel drive low range (4WD LOW)
• Neutral (NEUTRAL)
This electronically shifted transfer case is designed to be
driven in the two–wheel drive position (2WD) or fourwheel
drive position (4WD AUTO) for normal street and
highway conditions on dry hard surfaced roads). Driving
the vehicle in 2WD will have greater fuel economy
benefits as the front axle is not engaged in 2WD.
For variable driving conditions, the 4WD AUTO mode
can be used. In this mode, the front axle is engaged, but
the vehicle’s power is sent to the rear wheels. Four-wheel
drive will be automatically engaged when the vehicle
senses a loss of traction. Because the front axle is engaged,
this mode will result in lower fuel economy than
the 2WD mode.
When additional traction is required, the transfer case
4WD LOCK and 4WD LOW positions can be used to lock
the front and rear driveshafts together forcing the front
and rear wheels to rotate at the same speed. This is
accomplished by rotating the 4WD Control Switch to the
desired position. Refer to “Shifting Procedure” for specific
shifting instructions. The 4WD LOCK and 4WD
LOW positions are designed for loose, slippery road
surfaces only. Driving in the 4WD LOCK and 4WD LOW
positions on dry hard surfaced roads may cause increased
tire wear and damage to the driveline components.
NOTE: The transfer case NEUTRAL position is selected
by depressing the recessed button located on the lower
left hand corner of the 4WD Control Switch. The transfer
case NEUTRAL position is to be used for recreational
towing only. Refer to “Recreational Towing” in “Starting
and Operating” for further information.