Wierd shifting transmission

Ok folks, this is the first time i have ever asked a question on this forum so if I ask something stupid or post this somewhere where it's not suppose to be, go easy on me.

Heres the problem, I have a 92" Cherokee laredo and this is number 17 of 19 I have had and this is a first for this problem. It has 98000 on it and the transmission is having shifting problems. Usually when I start off, I can drive for about ten minutes, i'll stop for something and when I return to the Jeep to go again, it sticks in first/second gear. The first time it happened it corrected itself within the matter of a minute. The second, it took ten minutes, the fact that i hit a bump at the moment it started to shift again had anything to do with it...i don't know. The third time it happened and i immediatly clicked the shifter into 3rd and thats where it stayed. It seems to shift between second and forth but will not go into first. Transmission shop says the tanny is fine, it's an elec. problem. Please HELP!!!!!!!
P.S. Where is the Powertrain control module located?

First, start by checking the TPS sensor on the throttle body. That's usually the culrpit for erratic shifting. An analog meter is the best way to measure it and make sure it's changing smoothly as you open/close the throttle, but you can get some ideal if it's bad using a digital meter. They are mechanical just like an old style volume knob and do wear out and get scratchy. They also hate being power washed.

If the TPS checks out, then unplug the Trans Control Unit (TCU located under passenger dash under the glove box) and measure the resistance of the solenoids to ground. Pins C14, C15, and C16 should be 11-15 ohms to ground. Way outside of that and it's probably a bad solenoid. While you're there, ensure you have power on the yellow and red wires with the ignition on.

Another remote possibility is that the rear output speed sensor is bad, but they usually just fail altogether and the tranny never upshifts.

I doubt its the TCU, as I've only heard of 4 confirmed bad ones. If you're really convinced it's the TCU after checking everything else, I have a few spares. I'd have to check if I have one that'll work in a 92, though.

Being a 92, versus a Renix (87-90), I doubt the TPS is his problem. The vehicle speed sensor and the tranny solenoids came to mind, but what about the ECU to TCU signal that replaced the TPS to TCU signal in 91 and up?