If you have not done any injector maintenance yet, you should try a bottle of injector cleaner in the fuel tank. I like Red Line SI-1. A chemical decarbon might not hurt either.
But the fact that it started happening after changing the plugs may still be significant. Sometimes ignition misfires manifest themselves in strange ways, even with RPM specific or light load misses. Did you look at the plugs again? Did you crack one when installing it (this sort of miss can be from a cracked plug)? Did any of the plugs pick up any strange deposits? Did you try substituting a know good coil?
Lastly, it could be an out of balance problem. The first harmonic for a vibration to show up from something out of balance is 1700 RPM. What accessory on the engine it turning at 1700 RPM when the engine is at 2300 to 2500 RPM? Alternator, power steering pump, air conditioning compressor, water pump? Running the engine briefly with the belt off would confirm all but the water pump.
