Researchers have had some doubts about this. But a recent study found that people with persistent whiplash (three months or more) have a lot less neck movement in all directions than healthy subjects.
Amount of neck movement may separate people who have whiplash from those who don't. Neck movement correctly distinguishes people with whiplash from those without 80 percent of the time. This number goes up when age and gender are also taken into account. So yes, neck movement really is different for people with whiplash.