Quick thought, the only difference between how the diff is mounted in an mx5 & your roadster, is the brace that goes from the diff forward. I know this is too bulky, but, if you used a bar with female ball joints on each side, this should stop the problem. I think that the diff is twisting under strain, snapping the arms, hopefully the female ball joints should allow enough movement to take the load off the diff when you are pushing on.
