I would start with checking the most obvious thing first and that is the wheel nuts. Then I'd make sure the tyre pressures are correct before having the wheels balanced. I had a similar issue a while ago when I had new front tyres. I noticed a very slight vibration at certain speeds. The tyre fitter couldn't of balanced the wheels correctly cos once it was done again it was fine.
But as others have said it could be any number of things from tyres that have worn oddly, buckled rims, drive shafts etc. I also had a drive shaft go on a Mk2 focus st and that was horrible. Felt like a bloody wheel was gonna fall off!