The first thing to find out is whether the engine misfires at these speeds - if it does then we would suspect the injectors. You could try some injector cleaner as a first port of call. If the engine doesn't misfire then we would look at the following:

Are the tyres in good condition and do they need balancing? You could try changing them over from front to back (in pairs) and having them rebalanced.
If it's not the tyres then it could be that the front drive shafts are worn but if they do need replacing make sure on genuine parts
Alternatively it could be the propshaft balance as sometimes the balancing weights can detach or the engine mounts could be worn.

We would get these checked before considering the torque convertor.