I guess a little specificity is in order. I have an 01 SSEi and both left& right rear wheels became "squirrelly" at the same time. I'm not convinced it is even an actual strut/shock problem. There was a hole in one of the air lines and that assembly was replaced but did not resolve the problem. I can't get straight answers on what else can be done aftermarket and what is a dealer only "fix" . I'm not willing to be a $700 strut captive of the dealer unless I have to be. I'd love to hear from anyone with a similar model with experience with this problem

I haven't had any problems with my 00 model.
Our rear suspensions are identical. There are no struts in the rear. There is a trailing link type lower control arm which holds a coil spring and an auto leveling air shock.
I think you have a problem with either a faulty air shock or air system.

I have an 2000 and my rear end varies in height. Sometimes almost 3 inches above normal, and the driver'* side is a half inch lower, at any measurement. I measure from the pavement, to the inside of the wheel well. I measured this for a week and it varied most days. I never noticed it, untill someone asked me why,or how I "jacked my back-end up".
I don't notice any difference, handling wise. I haven't looked into getting it fixed.

I am in contact with a pro in this area and he will be able to help I also work closely with my local dealership. GM is known for its ability to rip us off when it comes to struts.

Ty