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.
For future reference (unneeded, I hope!):
What is the tool? A flare nut wrench?
Can obdII distinguish a stuck or dirty injector from an open injector?
Did you check with a meter to confirm the injector was open circuit?
If the injector showed up in OBDII with a code for "open", I assume it means the coil is burnt out or there is a connector problem. I myself would check the injector with a meter for continuity before pulling it. If the injector coil was indeed open circuit, then injector cleaner is not going to help.