Now here's a theory, given that this problem isn't solely confined to Locosts but exsists in MX5's too, maybe it's not the mounting of the diff that causes the problem (OK, if it's allowed to move around too easily then it may highlight the weakness) but MX5's that have broken the diff have pretty much the same thing in common.
They have been involved in an accident.
The diff has a cast sacrificial point in it designed to break to prevent the PFF puncturing the tank. Now given that most of the kit cars using the MX5 as a donor are built from wrecked MX5's maybe we are inheriting the fault without noticing it.
As i said, just a theory
TK