You must have some record for number of responses, and I confess I had to jump to the end - which was your update.
The two things in your message that stand out are: the vibration is between 60-70 mph and it feels very clunky when going over a bump or road divider.
The 60-70 comment pretty much mandates that it's rotation related. Very typical of a wheel/tire problem.....or alignment. Looks like you've chased all of those possibilities.
But the clunking over road dividers has nothing to do with rotational speed - more to do with bushings or connection points.
Has the car ever been wrecked, or hit up front (sorry if this has already been discussed)? Any service history at all (have you checked with a dealer to run a report)? At this point, you need some outside help.
Every town I've lived in has at least one shop, usually a frame/alignment type shop, that can work magic. Phone a few body shops and tell them what you're up against, and ask where folks take their tough frame/alignment jobs. I'll bet you get consensus after a few phone calls.
It's worth a try.