Bump. Having same issue. Would love to know where you landed on this.
This only occurs at higher speeds, usually above low-mid 70's but sometimes as low as 65mph. The car will start experiencing heavy vibration that feels almost as if you just lost a low of air pressure. First time it happened that is what I thought it was. I pulled over immediately. I could see nothing wrong with the car. I expected I would have to limp with this vibration to an exit, but as I started driving again, the vibration is totally gone. Once 70+ is reached, rinse and repeat. If speeds are kept to ~65 or below, you never see it. The issue persists after the winter wheel swap, so that is not it. Importantly, the vibration remains at lower speeds once it starts. You have to come to a complete stop to "reset it" and make it go away. It seems like once it reaches a particular harmonics it sticks.
I suspect that is a rear axle thing because the wheel doesn't shake violently. Also, I recently replaced the front lower control arms and bearings and had the car aligned, so I doubt it's the front axle, but could be wrong.