Jay. I seem to remember somebody fitted a dash cam underneath the front of a 3.7 to show suplex after fitting I think. Wondered if you might know someone who would be willing to fit one to the underside of your car to see if it shows what is happening.
Just a thought.
I had the shimmy at around 55 to 60 mph on my 2008 Roadster as well but it is now cured with new tyres and wheel balancing with a cone. I also made sure all the front end bolts and fittings were really tight ( some were not )on the cross member mounting points.
Hope you can get it sorted.

Jim.