I had the problem happen to me after buying the trooper used after 2 weeks.
I also have a '99 Trooper and it was pretty easy to fix. I had come to
the conclusion that it would most likely be 1 of 2 problems. The first
and most likely problem is the TOD vacuum solenoid unit (which is mounted
on the front axel under a splash shield) Try testing they unit while still
attatched to the vehicle and running: One end of the solenoid pack should
have a vacuum on it when the TOD button is pushed, and the other end
should have a vacuum on it when the button is back to normal.
You can try taking the unit off (remember where the vacuum hoses were
plugged in) and try to clean them out- first with some compressed air,
then with some WD-40, then with some compressed air again. Here is a
website that someone put out there to help with your testing.
Once you have the solenoid off, you can use a 9v battery to test the
solenoids to see it the are switching the vacuum path correctly by blowing
through the hoses.
The other problem (the one that I had, which I found after going through
all the other stuff first because I didn't think to check it even though
it was right in front of me) is the TOD position switch- may even be
called a front axel position switch which is located also on the front
axel next to the solenoid pack. I knew mine was bad because one of the
two wires was broken off. In order to get it off, you must first drain
the fluid out of the front differential, then unbolt the 4 bolts on the
housing that the switch is mounted on, unbolt the old sensor/switch, and
install new sensor/switch (with a new copper gasket for the sensor)
remount the housing, plug in sensor/switch, and refill differential (being
sure to add anti-slip differential additive).