Jeep 2001 Grand Cherokee 4X4

I have a Jeep 2001 Jeep Grand Cherokee 4x4 I6 whose initial problem was the car would not start. I removed the battery and cleaned the terminals and connectors which solved the problem. Three days later, on a trip on the interstate (approx.19 miles), the O/D Off indicator came on, subsequently followed by Trans Over Temp warning light. We pulled off the road, turned the car off and waited 15-20 min. Restarted the car and both indicators were no longer showing. Car runs normal around town but will experience the same if driven over 19 miles. Lately, the Check Gauges indicator appears on startup or when brakes applied—oil pressure gauge is at zero. The Malfunction Indicator light is also appearing.

I did take it to two Jeep dealers--the first could not find a problem, the second said need a transmission overhaul costing $2K. Believes cooler could be clogged indicating further problems.

Other pertinent information.
I did have the transfer case seal replaced when I had the battery checked and oil changed on day 2 before the trip on the interstate with the first dealer.

How can tell if this is a electrical/computer vs a real transmission problem?