AU III rear end wandering

[nassi]

Hi all, sorry to make my first post a question but I've searched through this monster of a forum but cant find an answer.

Recently my AU III XR8 sedan has been handling very poorly on occasion.
If I hit an irregular bit of road it feels like I have a flat rear tyre as the car begins to shake quite badly in the rear. Its almost like someone shaking it from side to side. This can also be happen in left hand corners.
There isnt any noticable noise. Ive put it on te hoist but cant see anything broken or rubbing.
TIA
cheers.