2009 Mustang hit from rear by car going 55 mph......?
Something doesn't add up here. If the car that hit you was really going 55, it would have been a much more severe accident! Your car would be much more damaged and you would have worse injuries - and most of all, your insurance company and your lawyer wouldn't be wondering how you got injured!
Have you had any x-rays or MRI to see if you've suffered obvious physical injury?
Anyway, even if the other car was going much more slowly, it's still common to get whiplash injuries in this kind of crash. I'm kind of surprised your lawyer isn't on top of that - maybe you need another one.