How would you describe 'nervous' w/regards to the rear at 100 mph corners? Is it 'floaty' where the rear end is just feeling light?

I myself get this in some classic high speed corners in the Midwest like 'the kink' at Road America and turn 1 at Brainerd. I have come to feel this is more of an aero affect and cannot be fixed with alignment settings.

I recognize there are way smarter guys than me on this board, and am preparing to get called out if I'm wrong, but I don't think you will find the confidence you are looking for in a slight alignment tweak in this case.

 

'floaty' is a good way of describing it. it feels like one wrong move, or light tap on the accelerator and the back end will just go its own way (which it did one time)

 

may be a gt-wing on the back may help to stablise the rear?