Sometimes rough...
 

...and sometimes it's not. A w123, and what I am experience is very minimal rough running from 40 to 70mph. There is no doubt that the vibration comes from the passenger rear wheel.

The vibration has been around for 3K miles.

The tire is to the wear line. Are these old sweet-hearts sensitive to tire condition?

I was thinking tire or rear subframe bushings because everything else in the rear is new. (Not CV-axle but is ok) I guess the easiest thing to do would be to swap tires around and see the results, but I post for knowledge. I would like to be entreated by potential issues so as to begin an adroit trouble shooting process.

Yes, they are sensitive to tire condition. Try switching the wheels around and see if the vibration changes side. If only that one wheel is wear badly then it's time too look at the bushings in the rear end. And thing about a 4 wheel alignment.

Best to buy new tires if you can. Also if shocks are worn a little imbalance can make a lot of shimmying.

Shocks are 1 year old Bilstiens. I'll do the tire and see how it goes. I got a set of 3 off a parts car. Great tires with 95% treat. The 4th tire is something of worn out horse.