When I had a '93 Trooper, it started to happen to me after my alternator
went of the flitz (must have spiked the electrical system)...shortly after
my starter went (what a coincidence).
Back to your problem though... It seems as though mine did it when it was
damp and cold out (usually on startup)...It had nothing to do with the
transmission, (at least on mine) and everything to do with an electrical
problem (when damp out, after starting the vehicle, it seemed to sputter a
bit (thus if you started driving, the trasmission computer thinks you are
having a problem with the transmission and puts it into "limp" mode-
straight 3rd gear).
If you have a repair manual, it may have something in there on how to read
the codes (without a code reader) I think I had to short 2 pins together
on the plug on the Drivers side under the dash, then the light would flash
the code back to me- came back as some transmission solenoid. I didn't
think that was the case because it would only happen on a rough idle after
I read somewhere that many electrical problems on the Troopers happed from
oil "pooling" in the spark plug wells- IMO due to a manufacuring defect
that ISUZU is not taking care of, but that's another issue. Try removing
all of your sparkplugs and see if any are coated in oil (3 out of 6 of
mine actually were coated on the outside of the plug- where the boot plugs
on to them) Isuzu does sell a new spark plug wire set which is suppose to
stop the "pooling" of oil by sealing the spark plug well better with a
revised boot- I think the set was $60 or so from the dealer, but make sure
you get them from the dealer, as aftermarket may not be revised to current
Isuzu specs. also replace your plugs then and be sure to coat the wires
with dielectric grease (lots) otherwise you will not be able to get the
wires out without ripping them.
Hope this helps.