OMG ya it would be absolutely lovely if it was just a bad spark plug or something. They haven't been changed in a while, and i'd think it would be conceivable for the failing valve cover gasket to leak into the spark plug chamber and foul them up? What would I be looking for when inspecting the distributor cap and rotors?

And the vibration is absolutely god awful. If EGR's cause only slight or moderate vibration then I think that's out.