2 different sets on same vehicle

[rh71]

Before you tell me it's a crazy idea...

I have the Toyo Proxes S/Ts (rears are 315/35/20) and even after road-force balancing all 4 tires, I've been having a vibration in the seats while over 65mph only. The dealer put it up on a lift and spun the tires and it looks like the left rear is pulsating from the side view, so it appears to be out-of-round. I'm assuming it's the tires and not the rims (replicas) in most cases that cause this visual anomoly? I got these tires a year ago and only got around to looking into this now.

The problem is these are discontinued in favor of the S/T II.

IF I can't get a replacement from anyone, would it be crazy to go to S/T II in the rears only and leave the S/T in the fronts (275/40/20)? The tread patterns are different but I'm not sure if much else is. I didn't ever plan on having these asymmetrical/unidirectional tires rotated anyway. The vibration is driving me insane.