94 Cadillac Concours 100,000 miles, 4.6 Northstar

My husband's Cadillac Concours runs great until it warms up, (approx. 10 minutes drive time) then it fails to shift beyond 3rd gear. The fluid level is fine and no burning odor.

This happened on the highway without any prior warning. I managed to get it home by stopping for breaks to let it cool back down so I could go another 10 minutes. It was a slow ride home!

The message readout didin't indicate any problems and when I start the car it says everything is OK.

The local transmission shop said the problem is probably internal and quite costly. What does this mean? I'd appreciate another opinion.