I believe it is a performance rear sway bar. It's one my original mechanic bought and installed. When I bought the car for my son it had this problem.
There were so many things to fix, that the suspension was left for last.
It had on these cheap skunk sleeves and blown out struts. We changed the springs and struts thinking that would solve the problem. Alas, the problem was still there. Next, it was discovered that the front end links were bent. These were replaced along with most of the bushings, which were blown out. The problem still exists. New sway bar installed. Some
improvement, but still there. Alignments were done twice with front camber
negative by 1.5.

As you can see, I've gone thru this suspension one step at a time. What I
have done is get a new mechanic who may have a better handle on suspension problems. The car is now at the third alignment shop and hopefully they will be able to give me the true reason for the horrible
suspension. It has to be one of the three following problems:

bent frame
camber / caster / toe in or toe out problem
suspension component issue

Wish me luck! Thanks, Doug