1997 Ford Taurus engine or drive-train vibration

I have a odd vibration that has crept in over a month or two, almost feels like an out-of-round tire. Tires were replaced and road-force balanced, complete front end checked and aligned, shock/struts replaced, but it's still there. It's better at times, worse at others, but always seems the worst first thing in the morning. Local Ford garage said it needs catalytic converters (there's an check-engine code for this, has been for a while), but they did not road test it, just said that based on the code. Can a bad cat or two feel like a suspension wobble? There are no driveability issues - plenty of power, no hesitation, holds 70-75 mph very easily. I feel this the most at city speeds vs highway. Thoughts?
Thank you, Bob Shotwell [email protected]