First I would get a dipstick and get the correct level of fluid into the transmission, also do another flush to try to remove more of the burned in contaminated fluid. I would also check the transmission ground located at the front of the engine on the top of the timing chain cover there are two wires bolted to the cover and one of them is the transmission ground. Remove the bolt clean it up with a wire brush clean up the lug and assemble it with some anti oxidation grease it could be as simple as the computer not receiving or sending the correct signal due to resistance in the electrical circuit. I wouldn't get my hopes up though since the rest of the gears appear to be working normally.

 

Sounds like worn frictions, we have rebuilt quite a few RE4's and are starting to play with RE5's now. When we start breaking 300whp on the RE4's the High drum frictions are the first to wear out and can't hold under power in upper gear. With 180k I bet it's time for a rebuild or replacement. If you plan on driving it for a long time I would suggest installing a Transgo HD shift kit to take the slop out of the shifts and reduce the wear big time.