I have a very similar issue, although it isn't too bad anymore due to a few things that I have done. It's like a pinching pain in my right hip and it is usually aggravated by periods of sitting. Walking or excercise actually help, but if overdo it, then I will feel it worse once my body cools down. There is always the possibility that it can come back but it is really not too much of an issue anymore.
There are the things that I have done:
1) Improved posture (especially when sitting): This was by far the most important change I made and I felt an almost immediate benefit. I have a desk job so poor sitting posture has been an issue. At my desk I can move around and stretch when I need to, but in a car you are kind of locking into that position. For driving the best thing I did was buy a lumbar support pillow. Ideally your back should have a good inward curvature (towards belly button) while you are sitting, but most of us are lazy and let it curve outwards (slouching). The support pillow will help to acheive the proper form and also just remind you to keep good posture. You don't need an extreme curve, just a nice healthy one. I could instantly feel how far better supported my lower back was. You can even just roll up a small jacket or towel if you don't want to buy the pillow. Ideally you can just practise better posture with your back muscles, but sometimes you will get lazy and slouch. The pillow is a very good reminder.
2) See a good sports physiotherapist:You can see a doctor but I find they often don't provide a ton that is useful. If you can get the MRI then that would be good. However, the physio will advise you on better posture (standing and sitting) as well as give you some good excercises and stretches for working the right muscles. If your problem is different from mine, they should be able to figure out the specific issue. This is how I became aware of my posture issue. Just make sure you do your research find someone good.
3) Proper excercise:If you are doing any kind of heavy lifting, or even just bodyweigth squats, push ups or whatever, you have to really focus on posture (can't be meantioned enough). Maybe this isn't an issue for you, but if you excercise and your posture isn't perfect, then it will make the problem much worse.
4) General care:I bought the roller that I use sometimes (not everyday), and I go to more massages (deep tissue), and try and stretch more. None of these things necessarily have had a substantial effect on their own, but I know they all helped. If you sit throughout the day, then try a do a few stretches every 30 minutes or hour and even go on a few short walks throughout the day to loosen up. At massage, I had them work on my psoas muscle a few times, and I think that might have helped.
All of those things and just general sense is what worked for me. At one point it was pretty bad (thats why I went to the physio) and now it is not much more than a slight annoyance from time to time. Usually that can be attributed to be being lazy with my posture.
I hope that helps out. I tryed to make as comprehensive a post possible because I know how annoying (and sometimes painful) that can be, especially if you are trying to be athletic.