If the 2004 is like the 2005 you can shift from 2WD to A4WD or 4H on the fly. Here is the info from the 2005 manual:
Positions of the Control Trac system
The Control Trac system functions
in four modes:
• 2H (2WD HIGH) delivers power
to the rear wheels only. This is
appropriate for normal on-road
driving on dry pavement.
• A4WD (4X4 AUTO) provides
electronic control four-wheel
drive with power delivered to all
four wheels, as required, for
increased traction. This is appropriate for all on-road driving
conditions, such as dry road surfaces, wet pavement, snow or gravel.
• 4H (4X4 HIGH) provides mechanically locked four-wheel drive power
to front and rear wheels. The “4X4 HIGH” light will illuminate in the
instrument cluster when this position is selected. This position is not
recommended for use on dry pavement. This position is only intended
for severe winter or off-road conditions, such as deep snow, ice or
shallow sand.
• 4L (4X4 LOW) provides mechanically locked four-wheel drive when
extra power at reduced speeds is required. The “4X4 LOW” light will
illuminate in the instrument cluster when this position is selected.
This position is not recommended for use on dry pavement. Use this
position for off-road low-speed operation or when extra power is
required, such as climbing steep grades, going through deep sand or
pulling a boat out of the water.
Note: If your vehicle is equipped with AdvanceTrac with RSC, the
AdvanceTrac with RSC system will automatically turn off the stability
enhancement feature when you shift the Control Trac A4WD system into
4L (4X4 LOW). The brake traction enhancement feature will still be
enabled.
The AdvanceTrac with RSC stability enhancement system can be turned
off manually by pressing the AdvanceTrac with RSC button (refer to
AdvanceTrac with Roll Stability Control (RSC) Stability
Enhancement System in this chapter) while operating in 2H, A4WD or
4H while driving in deep sand, very deep snow or more strenuous
off-road maneuvers. This will disable the engine management feature,
allowing the vehicle to maintain full power and enhanced momentum
through the obstacle.
Shifting from 2H to A4WD or 4H
Move the control to the A4WD or
4H position at any forward speed up
to 55 mph (88 km/h). The “4X4”
light in the instrument cluster will
illuminate if 4H is selected.
Shifting from A4WD to 4H
Move the control from A4WD to 4H
at a stop or while driving at any
speed.
Shifting from 2H, A4WD or 4H to 4L
1. Bring the vehicle to a stop and
keep the brake pedal depressed.
2. Place the gearshift in N (Neutral).
3. Move the control to the 4L
position.
If any of the above shift conditions are not met, the message center will
display the appropriate information.
Note: Some noise may be heard as the system shifts or engages.
Shifting from 4L to 4H, A4WD or 2H
1. Bring the vehicle to a stop and
keep the brake pedal depressed.
2. Place the gearshift in N (Neutral).
3. Move the control to the 4H,
A4WD or 2H position.
If any of the above shift conditions
are not met, the message center will
display the appropriate information.
Note: Some noise may be heard as
the system shifts or engages.