Let me give some words that may lend some help to this matter.
I have a 2009 Challenger SRT-8 and for over a year I drove it at the ride height that came from the factory. Six months ago maybe more, I decided to change it up and put on coil over shocks and I lowered the car probably six inches or more. The fenders now sit at the tire and although it does not rub anywhere at 80 mph I have a small steering wheel vibration.
Like you it is only at 80 mph and at 82 it goes away I did not have it before but now I do. Being so low the car steering is not 100% aligned so it is my cross to bear for having the car so low. Perhaps your issue is in the suspension I know yours is not lowered but perhaps the front end is not 100% aligned.
You might want to start there. I do not think you said where you feel the shake? In the seat of your pants or in the steering wheel?