it could be your wheel bearings too ... mine sounded just like a rolling bowling ball when driving slowly (easiest to hear at such speeds) and I had some nasty highway vibrations 50mph+ too ...
My wheel bearings were actually seized and disintegrated inside hubs, so car was literally rolling on hubs (instead of hubbearings) in fronts ... still it had zero play on tires ... check for free spin on front wheels with the whole axle jacked up (and trans in Neutral). even rims were not getting excessively hot even after driving long distance ...
also another noise thing that sounds similar is severely seized calipers, metal pads rubbing rotors will sound similar, maybe less "gravely", but it will make noise too. do you feel like your car was feeling heavy (like towing e.g. a boat) or has less power than in past?
if the brakes are problem, then it's quite easy to tell (check free spin as above) as usually applying little brakes will stop the car on dime, dragging brakes make cars stop very fast

they will also make rotors VERY hot and looking black & glazed.