I have been thinking perhaps a shock absorber as well when I read the thread. Based partially on inspections finding nothing sloppy, tires changed and balanced etc.

I'd also take the tires off and roll them down the driveway, see if there is any sideways motion indicating internal issues even if they balance OK. Had this a long time ago, replaced everything, still had vibration. found out by accident when I rolled tire into the garage. It had a slight sideways motion every rotation. Its next rotation was into the trash.

How were the ball joints checked?

What about the strut mounts in the front? They are easy to overlook. I saw a set broken on a coupe that pulled into my Indy's shop about a month ago. The guy had no idea they were broken, but when Gary showed him-boy did his eyes light up!