The upper back or thoracic spine is a commonly injured area in whiplash injuries.  The most common problem is caused by a sudden extension or side bending movement which crunches the little joints in this area together.  These joints, called facets, come in contact with each other when you stand or sit straight, or bend backwards.  Because they're painful, patients often get into bad postures because standing or sitting straight causes pain in these joints.  You can help combat this by trying some posture exercises.   Also, you try some home acupressure techniques detailed at http://www.acu-press.com/forward.html.

Another common problem in the upper back is the rib cage.  The ribs attach to these back bones and any alteration in their normal movement can cause upper back pain.   This can happen more often from ligament injuries than from broken ribs.