It could be that they need balancing on the actual vehicle, a mate of mine had the same problem on his XR4x4.
Does the problem only happen around 50? I.e. does it smooth out above and below, if so that is more than likely balancing, but it still might be to check other things like bushes, bearings, etc. the bigger (?) wheels may have highlighted a problem that was already in the making.
Chris J