I've tried searching but having trouble getting a good keyword for this problem. Its a 94 Trooper, 3.2L with an automatic. The truck runs good, no slipping at low speeds or from a stop. When it hits about 25MPH it just feels like it goes right into neutral instead of making the shift to 3rd gear. If I press the winter mode switch with the selector in drive, it also revs like its in neutral. I've found some info elsewhere online that suggests a solenoid under the front trans pan, or possibly just a trans gone bad(burnt clutch pack, etc). Specifics are hard to come by though, so I'm asking the experts.

I have not seen a check trans or check eng light, but I also have not pulled any codes from it to see if any are stored. The engine runs fine, and has plenty of power and torque to the wheels, until it gets over 25MPH then it just revs free.

I think I remember reading somewhere that if you have problem like that with the 3rd gear then it is usually burnt clutches. Check the planet isuzu forums. There are a lot more trans articles over there.