I have just tried the Sierra header tank and cap and it no longer steams so I think this is Ok. Whilst trying it I drove for a bit in 2wd mode. Now these tyres are probably quite old but it wheel spins in 4th gear as soon as it hits 4000 rpm

So I have come to the conclusion that unless better tyres solve this (poss R888) then 2wd is purely for being a hooligan and smoking tyres.