Interesting, thanks Steve
The tyre balancer machine said I needed 255g on one bead of my modulars and 225g on the other so with 480g in total I don't know if 6oz is going to be enough, though I suppose the fact that they are at the outside of the wheel rather than on the rim must make the same weight more effective. One site I looked at said that for 33x13.50-15 you should use 8oz per wheel. If it gets rid of 80% of the shake then I will be OK with that anyway, and I suppose I could always get another packet and split it between the 2 wheels. Only thing I did see somewhere is that you are not supposed to use them in a tyre that has had a plug put in it, so I guess if you get a puncture it is either throw the tyre away or put up with the shake. I think the only thing that would go through a Mudzilla would be big enough to destroy the tyre anyway, like a fencing standard, so it isn't a big deal for me.
I think I will try some just in the fronts, and see what happens. If that works I may get some more for the other wheels. Sent an email to D4x4 to enquire about shipping costs and await a reply...