Rear wheel vibrating or hopping at speed

[econti]

To cut a long story short:
I've been chasing driveline vibrations since fitting LSD and refreshed driveshaft with all bits.
Everything diff and driveshaft is new or has been swapped out for a known good part, so that's now not a factor.
I have switched wheels and no change.

Basically what happens, is that between about 40-50mph there's a vibration from the rear end. Above that it seems to settle down a bit, but is still there.

So I put it on jackstands and had my brother drive it to that speed, and I saw that the rear driver's side (passenger side for LHD) wheel was kind of, rapidly hopping up and down. As if it was out of balance, but I know it's not.

What could it be? Bad outboard CV joint? There's no movement in the bearing itself that I can feel. I can't hear any grinding or clicking.
It's really hard to describe, and I had a video but you can't see it on that.

Could it be brake rotors?
Any help appreciated

[ahlem]

Did you swap the wheel and see if a different wheel did the same thing?
If you have a dial indicator and magnetic base, you can put the car in neutral and roll the rotor and check to see if something is out of round. You can do something like this with a coat hanger wire as well.

[econti]

Swapping wheels doesn't change anything. Had them rebalanced as well.

I do have a full set of new rotors that turned up yesterday. Will get pads and lines then swap and see what happens.