Hoefel,
First, if you are sitting on your wallet while driving, you need to remove it from your back pocket.
If that is not a problem, then the situation becomes a lot more difficult to ferret out. Especially since it only occurs when you drive your car and nowhere else. If possible, you need to determine what is different about your seat and how you sit in the car. What makes it so different? If you can figure out what is causing the problem, then you can figure out what part of the body is producing the symptoms.
But, it is going to be very difficult to tell what the problem is, until you can figure out why it only occurs in the car. You might want to see an orthopedic surgeon for a full examination of the spine and lower extremities. But, if you are not having any problems when you see the surgeon, the exam probably will not show anything.
You might also want to see a physiatrist, a specialist in physical medicine and rehabilitation. These physicians deal with musculoskeletal disorders and biomechanics. He/she may be able to evaluate how you actually sit in your car and what is causing the problem.
But, again, this is going to be very difficult to determine since it only occurs in one setting. What about your car is causing the problem?
Good luck.
Saving 
