Will a bad oil pressure gauge sensor cause your truck to run rough like it's falling on its face while downshifting? Or does it indeed have a probable transmission issue developing and should sell?

SOURCE: 92 ISUZU TROOPER No speedometer or odometer

It should do unless the gauges or boken/loose wire was the original fault

SOURCE: oil pressure sensor location

drivers side right above the oil pan

SOURCE: 2000 ISUZU TROOPER WITH AUTO TRANSMISSION. CHECK

If the fluid is up -
it could be a vacume loss or the shifting module..
The shifting module is what tells it to shift - and it is controled by vacume pressure..

SOURCE: low oil pressure 93 GMC P/U w/350 motor w/205,000 miles gauge

While engine is running, open the cover where you add oil to engine and see if you see a good flow of oil, if good flow, could be as easy as replacing oil sending sensor. got to realize that if poor maintenance history your engine may not be building enough pressure at iddle to move your hydraulic operated valves, only when you rev it up it will pump enough to run smooth. if running engine too long, you may cause more damage to other engine components such as crankshaft and cam shaft bearings. if you can spare a few bucks, have a good mechanic check your oil pressure with a mechanical gauge. another cheap option is to replace oil pump. last resort if you love your truck as much as we do,replace engine or rebuild it. At 205,000 this is a normal symptom, other than timing chain problems, most engines don't make it that far. hope this helps!

SOURCE: 2000 isuzu trooper: OBD-II Trouble Code: P0401 Insufficient EGR F

Your SUV is really giving you a hard time, lol. Sorry but I've had those days too.
I would try to check if you have blocked EGR passages. If ok check for restricted MAP sensor vacuum hose/passage or restricted exhaust. I know you replaced the CAT but if the muffler is blocked then there would still be a restriction. Also check the wiring and connection to the MAP sensor.