Ford 2013 F-650 Owner's Manual page 146

It is not recommended, or approved, for axle conversions to be
performed. However, it is understood that, on occasion, aftermarket
add-on axles are installed by others on the truck chassis which allow
operator control for weight transfer from other axles (i.e., air lift axles).
TWO-SPEED REAR AXLE (IF EQUIPPED)
WARNING: Never shift a two-speed axle when descending a
steep grade as this may cause loss of vehicle control and result in
personal injury.
Note: Do not shift between ranges when the speed control is on.
2SPD
LOW
HIGH
Axle Shifting with a Manual Transmission
To downshift, select the next lower gear, release and press the
accelerator pedal rapidly, or while holding the accelerator pedal down,
release and engage the clutch rapidly. Note: The clutch method is
recommended when driving at slower speeds.
To upshift, keep the accelerator pedal down, select the next higher gear,
release the accelerator and pause until the axle upshifts. Note:
De-clutch for smoother axle upshifts when driving at slower speeds.
Axle Shifting with an Automatic Transmission
Note: You cannot split-shift with an automatic transmission. Also,
downshifting above 40 mph (64 km/h) may result in transmission or axle
damage.
Note: Do not shift the axle to LOW with the vehicle in motion.
Use LOW when you drive a fully-loaded vehicle on a severe grade or in
congested traffic. Press the upper portion of the switch.
Use HIGH for all normal driving conditions with a lightly-loaded or
partially-loaded vehicle. Press the lower portion of the switch.
2013 F-650/750 (f67)
Owners Guide gf, 2nd Printing, March 2013
USA (fus)
Transmission
A two-speed rear axle allows the
driver to select a low range for
greater pulling power and a high
range for greater road speed and
fuel economy. These ranges can also
be used to provide additional steps
between transmission shifts when
driving on steep grades or fuel
economy may be factors.
145