Transmission will only shift into first and reverse

Will it shift into those gears if you manually move the lever and only not shift up if left in drive or will it still not shift either way?
If it will shift manually into those gears but not while in drive it is likely a wiring issue or a failed solenoid pack in the transmission. If it will not shift manually then it is likely that there are broken seals internally that are bypassing fluid so it will not shift into the higher gears.
I would check fuses FL-3 and C-14 in the dash fuse box as C-14 powers the transmission control module and FL-3 powers the control relay.

Thank you Steve, I am thinking about buying this 1993 Isuzu Trooper that has been taken care of. These folks seem to think it is something electrical, over their heads. I have not driven the Trooper. I can buy it for $800.00. I am able to risk it, like so many before it, even sight unseen. Electrical problems is not an issue for me. So I had to look into it from the experts. I will take your advice and I will write back and let you folks know what I needed to fix it. Thanks, Tom Atlanta

Could very well be the solenoid pack or its harness. Hard to say fore sure without seeing what it does/does not do. If it is in that nice of shape I would grab it, worst case a rebuilt transmission is not a hard swap.
Keep us in the loop please.