The vibration seemed to get better for awhile, so we didn't bother with the injector cleaner. Then a few weeks ago, after a 30 minute drive on the highway the engine light started flashing and I experienced a rumble from under the front wheels. Fortunately it was only 5 minutes to home.
OBDII indicated #3 cylinder misfiring and injector open. I decided to replace all four injectors as they're only $40 each (Motorcraft CM-5148) Fairly straightforward but I recommend you buy the special tool to disconnect the fuel line from the fuel rail. They're about $5-10.
Car is running like new now. Smooth as silk.