Rear end sway

Let's start with the tires. Have you swapped them to the front or inspected them for wear? The most likely cause of this is the tires.

https://www.2carpros.com/articles/how-car-tires-work

If the tires are new then we need to look at suspension and have an alignment performed. I attached a bunch of info about the rear suspension but we need to focus on alignment and shocks or springs.

Let me know what you find and we can go from there. Thanks