Having bad vibrations in my 2015 Sport Autobiography, everything is fine until I get to about 65mph then the cabin vibration starts, no vibration on the steering wheel just the cabin. When I drop below 60 the vibration stops, jab the wheels checked and balanced to no avail. Has anyone else had this problem or any idea what the cause could be.

Well, similar things happened to me first on Chrysler 300 years ago, I was frightened that time. It happen to be that one emergency brake shoe was partly seized. After cleaning and readjusting problem gone.
Also, sometimes on 2006 RRS. for short period above 90 km/h, than vibration was slowly going off. I was thinking either rear brake cylinder or EPB . Than, after short time EPB warning came on with a little noise from back. EPB shoes replaced (thank's Bodsy for your brake bible) and adjusted, no problems since than.
I am not familiar with 2015. but you might take a look at rear brakes (cylinders, pads...)