What sort of tyres are you talking about? Mine are 31 mud, not that bad. I guess a set of 32", maybe Goodrich Mud of Goodyear MTR would be better... Forget about superswampers, terribly expensive. And bigger tires just don't look too good. I've seen Monty's with 33", and even with military 9-0-0 tires, and the style changes completely.

Anyway, I think that once you loose traction because of differential, there is no tyre that helps. In my case, even with the best tyres, all I would have moved is air.

Ah, and forget about me buying a diff lock, way out of my league. I have other priorities for my Monty.