The exact same thing is happening to me.
I am getting and have been getting it for awhile, noticeable lateral movement over the rear tyres, only on certain uneven surfaces.
I have read on these forums of that being a hicas issue. BUT this is server swaying and only happens occasionally.
Took it to pedders, they were nice enough to hold onto my car for an hour or two, do a few 'blockies' and proceed to tell me it's all good. (Didn't surprise me when he didn't ask to be paid)
Also on a side note my rear right is toeing in with a few degrees of negative camber(can take a photo) and I am only seeming to spin the rear left wheel now too.
So it's off to a new suspension shop and try to get someone with an idea to fix my car then the diff than the......