Thanks a lot for all the details. I wasn't aware of those things, so
I'll look 'em up- like what the heck Control Trak is. ;-)    
-Paul

This is interesting stuff. I found an excellent explanation of how
Control Trak works in an article on how to add a switch so you CAN lock
it in 4H:

There were three slightly different systems included in the
second-generation Explorers. The All Wheel Drive is a full-time system
and cannot be modified to be full-time four-wheel drive or two-wheel
drive. The second and third systems used in the '95-'96 and '97-'01
model years were both called 'Control Trac 4 Wheel Drive' Both systems
used the (insert model here) transfer case. The '95-'96 used a front
axle equipped with a Center-Axle Disconnect, which was done away with
on the '97 model year in favor of a live axle (and fewer parts to fail
when needed most)

Depending on the operator selection, the transfer cases in both systems
were actuated by one of two GEM-controlled sources: The Torque On
Demand Relay and the Transfer Case Shift Relay. The transfer case
contains a Transfer Case Clutch Coil, which is what locks the front and
rear driveshafts together. Power for the TCCC is fed via a brown wire,
hence the familiar name. The Transfer Case Clutch draws approximately
5A of current. Make sure that all the components used in the wiring and
switch are rated for at least 10A to be safe.

Some more specific background on the two systems.

'97-'01 (switch says, 4auto, 4high, 4low)

The '97-'01 model years lack a true 2 wheel drive option. With a live
front axle, there are no significant mileage gains to be had by driving
the rear wheels alone. That being said, some of us simply prefer to
determine when the front wheels should drive and when they shouldn't,
instead of relying on a computer to make that decision for us.

In 4 auto, the GEM monitors the wheel speeds. When it determines that
there is wheel slippage, it sends the signal to the Torque On Demand
relay to actuate the Transfer Case Clutch Coil via the brown wire. By
interrupting the power flow to the Transfer Case Clutch Coil, you
prevent the front driveshaft from locking to the rear, hence a true two
wheel drive.

Now, please note that the TOD relay is either on or off, and the
Transfer Case Clutch is either engaged or disengaged. The relay and GEM
do not vary the amount of power applied to the transfer case, they
simply vary the DUTY CYCLE of the TCC. The transfer case is either
engaged or disengaged at any given moment.

When you select 4 high from the switch on the dash, the GEM sends the
signal to the Transfer Case Shift Relay to engage 4 high. This sends
power directly to the Transfer Case Clutch Coil, essentially a 100%
duty cycle.

If you select 4 low, the transfer case remains locked, but engages a
set of planetary gears to obtain the low range gear reduction. The
front and rear driveshafts are still linked solely by the Transfer Case
Clutch. By interrupting the power flow to the TCCC when in 4 low, the
transfer case disengages the front driveshaft, even though the
planetary reduction gears are engaged. This results in a true 2wd low
range.

'95-'96 (Switch says, 2WD, 4auto, 4 low)

The system used in the early second generation Explorers was very
similar to the later second gens. The '95 and '96 model years used the
same transfer case as the '97-01 in conjunction with a Center-Axle
Disconnect on the front axle. In 2wd mode, the transfer case is
unlocked, and the CAD essentially breaks the passenger side axle shaft
between the front differential and the passenger side halfshaft. That
allows the front differential to stop turning, leaving the driver's
side wheel to drive only the spider gears inside the diff.

When you engage 4auto on a '95-'96, the GEM switches the signal to a
vacuum solenoid (insert picture here) which locks the CAD, thereby
connecting the front half of the drivetrain and forcing the front
driveshaft to turn with the wheels. Then, when the GEM determines that
the rear wheels are turning faster than the fronts, it momentarily
locks the transfer case, linking the front and rear driveshafts
together for a period of time. The GEM then unlocks the transfer case,
checks for slipping again, and, if necessary, locks the transfer case
again. This produces some clunking and unconventional engagement and
disengagement that many people don't like.

To combat this, we simply bypass all the GEM-controlled electronics
and, using a simple switch, apply power directly to the transfer case
clutch coil, again via the brown wire.

4 low functions the same way as in the '97-'01 models. The transfer
case engages a planetary gear set, providing the low range gear
reduction. Interrupting the power flow to the TCCC will, again,
disengage the front driveshaft, providing you with 2 wheel drive low.