When I removed the exhaust hangers at the cross member the vibrations disappeared. It may look like the exhaust pipe is near the bottom of the hole in the crossmember, but it’s just the angle of the shot. There is a 1/4″ or more of space.
Since the hanger wasn’t fully insulated from the crossmember I decided to open up the bolt holes and insert rubber grommets to isolate the bolts from the metal hanger.
I then added reinforced rubber to each side of the metal hangar.
Unfortunately this didn’t work as the bolt threads cut through the grommets.
Next I went all out and enlarged the bracket holes to accept a short length of gas line hose and also added two more rubber spacers. I needed to replace the original bolts with longer units. This done and installed it didn’t seem to make a whole lot of difference in the harmonic vibrations.
Next: I need to remove the brackets and then do an actual test drive to verify that the vibrations are indeed gone and that the hangers are the problem.