1994 BMW 525i Feels like the rear end is shaking

I was test driving my wifes car and it feels like the rear-end has this kinda shaking, waddling, feeling. I don't think its the front end because I don't feel any shaking or anything in the steering wheel. mostly under my seat. It doesn't feel like a vibration, more like a shaky, shimmy, waddle issues. Sorry for the lame description just don't know any other way to put it.

I plan on taking it a shop for repair but was hoping for a bit of advise going in.

Thanks