My D2 did it, my D3 did it, my D4 does it - all on standard wheels, either summer or winter tyres. My S4 does, all of my S4's have! Sometimes worse than others, never really correlated it though.
The diff locking makes some sense though, I could be talking twaddle but I guess that you normally want the diffs to lock when both wheels have lost traction - but that when they do lock then if you still have some traction you get the judder. So on ice, mud etc no issue, but a bit of slip means you feel the judder.
In that scenario the perfect storm could be cold weather, tyre wear, wheels at lock (camber), wet surface, shiny car park surface - or any combination of the above???
edited to add, I don't see it as a fault, just a quattro characteristic
