1998 Cadillac Concours fails to shift beyond 3rd gear

1998 Cadillac Concours 120,000mi 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 didn'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.