92 Crown Victoria transmission slipping and Vibrating

I recently bought a 92 Crown Victoria from my grandmother that only had 20,100 miles on it. It looks perfect and has been garage kept and Dealership maintained. The only problem was that the transmission vibrated a little going into overdrive. In the past month I have put 300 additional miles on it hoping the problem would work itself out. But the vibration has gotten worse and now there is some slipping and transmission gear indecisiveness. I dropped the car off at a shop, I have trusted in the past, and the mechanic just drained a quart of tranny fluid and added a Lucis transmission additive. He argued the tranny didn't need service until 30,000 miles. Is he right in this instance? Have put another 200 miles on it since and its a little worse. Any Ideas? Could it need a new tranny with only 20,600 miles?
Thanks