The fact that the vibration fades in and out suggest to me that it is not balancing. These days balancing is all done automatically on a machine, so it is hard for even the thickest work experience boy to get it wrong. The variation in frequency with speed and the fading in and out may point at a resonant frequency being set off at various different speeds. To me this suggests suspension mounting bush wear or any form of rubber bush mounted at the joints in the rear axle/suspension links.

The onset of vibration fading in and out is caused by two frequencies not quite the same (like two props on an aircraft going at slightly different speeds), so that explains the variation due to speed, but also that the (for example) suspension mounts could be worn different amounts and therfore have different resonant frequencies (same as a glass humming when you rub the rim).

I'd get it into the garage for a look.

Hope it isn't serious.