What is the mileage difference between the front and rear tires? Same type of tire? What kind?
I ordered some Toyo R888R's a while ago, had to order fronts from a different place than rears due to availability. Fronts came in first and I swapped them out before driving from Houston to Austin. That was a terrifying trip. Rears were the original Super Sports, had 10k miles, fronts were new. Every time I went WOT at freeway speeds, the car felt squirrely as hell. It was like the rears were getting loose while the fronts were grabbing hard, which may have been the case. Put the rears on as soon as they came in and the problem went away. Car is stock. Not saying it's the same issue as what you're having, just relaying my experience. Good luck.