2003 SAAB 9-3 vibration after 70mph

I had brake rotors and pads, one control arm and tie rod replaced there where bad. Also had front tires balanced as well. When driving on high way over 70mph I feel a vibration on steering wheel no wobble the sound is not loud tires are good year used but have a lot of tread, the rear tires are worn some tread left a little bald on the edge. So what can I do?
have rear tires replaced, balanced and switch them from front to back?
or can it be axles or wheel bearing? I have read some where that shocks can also cause this vibration