Jeep Cherokee Forum - One more for the tranny professor

Let me try to keep it simple, short and helpful for others.
Jeep Cherokee Sport '98, 6 cyl., 4.0 L, autotranny Aisin aw-4, 231 transfer case, 93,000 miles.

When engine is in normal temperature range the tranny doesn't shift from first to second gear. RPM get high (over 4,000) and the only way to make it shift is to momentarly let go on the throttle. All the other gears and shifting is totally normal. The check engine light (MIL) is on.

Scanned the car and found code P0751. Looked at the Cherokee repair shop manual and the description is: "Overdrive Off Switch input too low for more than 5 minutes" for car equipped with a 4 speed automatic transmission.

Started from the basic and checked: transmission fluid level, adjusted the throttle valve (TV) cable, checked TPS voltage (cold and hot), checked transmission speed sensor, flushed transmission cooler and lines and replaced fluid and filter.

Went into more specific checks: disconnected TCM and drove it manually, everything was OK cold and hot. Manually the trasmission works perfectly.

Replaced the three trasmission solenoids, swapped the TCM with a friend of mine Cherokee Sport '98 identical car and nothing, the problem is still there. :brickwall:

Has anyone ever heard of anything like this before?
Was anyone brave enough to pull and overhaul the transmission valve body?

Any thoughts, hints and suggestions are very welcome.
If I ever solve the problem I will post the detailed solution.

PS: Our local dealer has no clue! :dunno:

Thanks.