1998 Isuzu Trooper Trooper transmission won't upshift.

You guys have helped me before with a range mode switch issue so I'm back for more. The same Trooper I listed before (I'll provide the particulars again at the end of this post) will no longer shift into 3nd gear. Beforehand, I had the dreaded range mode switch issue so I removed the old one, cleaned it out, put in new dielectric grease and had the Isuzu dealer flush the auto transmission fluid just to be safe. There have been no leaks and everything was fine and shifting like new for well over two months until out of the blue it started the hard shifting again, just like the last time. This time the new behavior was the transmission light flashed ONCE and never came back on. It was at this time that it developed the shifting problem and wouldn't go into 3rd gear. I thought "oh great, it's the range mode switch again". This time, I replaced the range mode switch with a new one from the Isuzu dealer, while under the truck I observed no leaking liquid from the trans or engine (no oil spots under car or on the chassis). I read about a "limp" or "safe" mode the trani can go into but I also read you should still be able to shift manually. I tried it and while I can get it to shift down into 1st, I cannot get it shift into 3rd gear. The best way to describe the behavior is to say that someone opened up my transmission, removed 3rd gear and up, and put it back together. The modes on the shifter, light up as expected (P, R, N, D, 3, 2, L) (might have the order wrong, but you get the idea) and they don't jump around. Drives in reverse and D just fine as long as you don't expect to hit any gear above 2nd. Again, it simply won't shift into 3rd or higher, the engine just revs. No smokey smells, no weird noises, no grinding, no hesitation. It's like someone just removed gears 3rd and higher. Well, I hope this gives you enough information. Oh, I also reset the ECM by removing the 30A fuse for ~5 min while I checked all the fuses to make sure none were burned out (all were fine). I'm handy with cars but transmissions are beyond my meager skills.

Isus Trooper
1998
3.5L Enginer
All Wheel Drive, low/high 4wheel manual
Auto Transmission
<100k miles