Got exactly the same issue with mine, side to side rear wobble and a rear end which feels it's made of jelly whenever you try and push the car on a bit. I'm just short of 100k on mine now and since I've had it 10 years I'm going to do a full rear suspension refresh.
In the past couple of years it's had a set of bc racing coilovers , eibach camber arms and spl droplinks (plus updated arb) and a recent alignment so I'm thinking it's a bush but again, nothing is obvious. I'm just holding off for the January sales in case i can get any bargain parts then will be cracking early jan with the refresh of all the bushes and arms. Will let you know how I get on.