'85 Volvo 740 transmission question

Just recently, my 1985 Volvo 740 stopped shifting into overdrive, and just continues to stay in a low gear. It's an automatic transmission, and we can't drive faster than 30 miles an hour. It will not shift into the final gear. I checked the transmission fluid and it seems to be O.K. Any thoughts on what could be wrong? My husband told me today that the "kickdown cable" appears to be frayed a little. I have no idea if that even matters or not. Help?!?