FORD 2002 Explorer - Manual (Page 204)

Using the Control Trac 4WD system
4X4 AUTO - Power to all four wheels; used for normal street and
highway driving.
4X4 HIGH (4WD High) - Used for extra traction such as in snow or icy
roads or in off-road situations. Not intended for use on dry pavement.
4X4 LOW (4WD Low) - Uses extra gearing to provide maximum power
to all four wheels. Intended only for off-road applications such as deep
sand, steep grades or pulling heavy objects. 4X4 LOW (4WD Low) will
not engage while the vehicle is moving; this is normal and should be no
reason for concern. Refer to Shifting to/from 4X4 LOW (4WD Low) for
proper operation.
Driving
204