My prop shaft bolts caused a vibration at the L7 Brands track day a few years ago - huge vibrations down Pilgrims under acceleration... I slowed right down immediately and it tended to go away, but didn't make it back to the pits as the propshaft bolts snapped somewhere around Dingle Dell, and the shaft came through the footwell.
If you are still driving it at 85mph with the vibration, you're very brave! 😬
I would have thought that if it was the propshaft bolts though, you'd be able to see the movement, and as it's only happening at about 85-87mph, it's not likely to be them. I have one solution and that would be to drive above 88mph.