I actually developed a lot of injury problems related to this, so I understand the concern. First, I'm not sure how you know you have a problem with pelvic mobility. It might be easier to watch yourself walk in a mirror than run, or to have someone else watch. Do your hips rotate at all? Try to walk with an exaggeration of this movement, really throwing each hip forward as you walk. If in doing this you feel you lack a free rotation of the pelvis you could have some sort of mobility issues around your lower back and pelvis. There should be some good PT exercises you can pick up to help with this. I actually went the route of getting some therapeutic massage to unlock some of the small joints in the lumbar and sacral segments of the spine. I'm sure that there are approaches involving yoga or rolfing or whatever, but I've never tried them.
The other part of the equation is having enough abdominal strength to drive that hip action throughout a run. For this you have to build up your core abdominal strength. It seems like every workout program now is about core strenght, so whether you use an excise ball or whatever, you need to concentrate on these workouts. Especially exercises that emphasize a twisting movement which will mimic the pelvic rotation you want to develop.
The last step will be to concentrate on actually using correct form when running. Pay attention to your running stride and actually concentrate on generating forward drive of your legs with your hips, rather than focusing on driving your knees forward. Pay attention to your upper body. Are your shoulders rotating? Usually if your hips are not moving your whole upperbody will not be rotating either and you'll be rock solid all the way down to your legs. You do not want this. In addition to focusing on using your abdominals and driving forward with the hips while running, do some drills specifically designed to emphasize this movement. bounding exercises where you emphasize generating power from your hips, etc.
Good luck.
JY