High pitch sound coming from rear wheels while driving?

It may quite possibly be the tires. At low speed there will not be a noticeable sound, but at higher speeds the contact of the tires' tread with the pavement will sometimes amplify what was once a barely detectable sound into the high pitched tone that you are hearing. Other possibilities could be at the brake calipers. The wear indicators on your rear disc brakes (if you have them) could be in contact with the rotor thus causing the sound. The only other possibility that I can think of is the wheel bearings. If I were you I would first check the tires though. If you purchased new tires this could very well be the cause.