Wow - You could be onto something and I have learnt something new
Thanks guys.
Yes, I tried a couple of thingsthis evening before stripping down the diffs. I removed the front dog bone, same problem. Replaced that and removed the rear one - Same problem. So I deduced it was something with the slipper. I loosened that right off and the judder went away (but with loads of slipping)
So I guess it is cogging (yes its a sensorless motor). Nothing to worry about then and no need to tear down the diffs (although I will look at flipping them over)
Thanks chaps - has put my mind at rest