Messed up rear suspension after tire change?
 

So this could be a total coincidence, but I just had Discount tire replace my rear tires (Kumho SPTs). I watched them do the whole thing, they jacked the car's rear up by hand, using the two pivot points of the rear toe links furthest back, near the center of the car just in front of the muffler.

Afterwards, at speeds 70+ the rear feels really wallow-y, sloppy, and takes a while to settle if i make a few quick shallow turns, such as a quick lane change. It is almost as if the rear sway is gone, or perhaps the shocks are bad? The only thing the tire shop could have done is accidentally push up on the rear links, causing them to pull the back part of the hubs in, resulting in a toe-out position, although I really doubt that would make the car feel like that.

Not sure what is going on, but tonight I'll lift the car up and take a look underneath....

Those are not really good jack points but I still think it should be okay. If they messed anything up it should be pretty obvious.

you can check your toe by looking across the back tire and see it it lines up with the front wheel on both sides. Then get 2 tape measures and 2 straight edges and measure the distance between the front and rear of both tires they should be the same distance for zero toe which is a good setting you definitely don't want more then 1/4" toe in or out.

I looked under the car, everything seems ok at first glance... One of the shocks is covered in grime, the other is pretty clean. However, the grime isn't greasy, more like black dust. Still though it could be bad...

It could just be coincidence that a shock failed around the same time you got new tires. New tires very well could have made it apparent. A change in handling is a common sign of failed shocks.

So i just swapped front/rear wheels. With the new tires on the front, it feel totally fine, like it used to. wtf? maybe the sidewalls on the new tires are not stiff enough? The new tires are the exact same model as the ones they replaced.

I guess I just gotta put a couple thousand miles on the new ones....

New tires can enhance a bad alignment. As said, you should have that looked at. So you only bought two new tires? That can cause a change in the handling of the car also.

Mismatching tires can also affect handling and alignment issues.