Re: quaife differential ??? $$$$ (vr6a291)
About $900 give or take a few bucks. Plus, you have to dismantle the transmission to install it, hence unless you are really mechanically inclined and have access to tons of tools, you'll need to pay someone about $350 to $500 to install it (depending on where you live and who you know).
Is it worth the money? I'll say yes, especially since the MkIIIs do not have any kind of brake based differential locking system at all.