Okay, sorry it took so long to come back but I solved the problem.
Hairline fracture in the LR K1 part, never seen it before.
So basically, under stress this part was obviously stretching and flexing in the alu housing (vertically I assume). The fracture is impossible to see when K1 is installed and "just about" impossible to spot when it is removed. Even the bearing still "clips" into place...
It was, unsurprisingly in retrospect, causing an imbalance across then entire rear diff.
Now that it's fixed my 511 is rear-end planted as we all want for that extra second.
Thanks for all the suggestions!
