man that sounds like your rear is falling apart..just kidding
i ain't E36 specialist, but have you really checked your shocks, check them for leaks, if they have never been replaced since out of the factory, maybe its time to do so, also the mounts on top of them check those for cracks.
could also be your rear bearings.
don't you have any records from the previous owner or did he/she at least tell you what work has been done. i know telling is like fairy tails but it never heards to hear it anyway.
Good luck..more pple here will give you input that's for sure