ORIGINAL: everspeed
I believe what's happening is when you hit, the plastic support is flexing too much, plus the screw bending, putting too much pressure on the tabs sticking up on the diff case. If the rear brace was replaced with the one above, it wouldn't give much, thus putting some of the shock on the chassis brace, and taking alot off the rear diff tabs.
Cheers Everspeed! That is a nice theory you have there and that's exactly what I think is happening, I'll buy that PCR brace before buyign the shock towers, (purchase will be at the end of the week when LHS is back off holiday).