Have you tried to go faster to see if it stops? Wheels are usually worse at ~50-60km/hr depending on their size (interestingly this corresponds with road corregations). I had a tail shaft on an old holden start vibrating once and that was very specifically around 95kms/hr.

If it just gets worse with speed and doesn't get better then you've got a large out of balance force, i.e. perhaps a damaged gear or an out of round drum brake or something. I've had worn suspension components and never noticed vibration but where I lived then the roads were pretty rough (and still are).

[Edit: another point if it is speed related you can try the same speed and a different gear (if safe) to see what half of the drive line it is.]