@wilcy My thoughts are the physical position while driving and lack of opportunity to change position might be causing the cramping. There can be other issues too like dehydration, etc. I know from personal experience that my pelvis can go out of alignment and when I have overly tight muscles in front of my hips, it causes problems, and nerve entrapment can happen around the pelvis with muscle that is too tight. Myofascial release and physical therapy can help if it is a physical issue. Here are some links and our discussion on Myofascial Release. I do MFR and it helps me a lot.
https://connect.mayoclinic.org/discussion/myofascial-release-therapy-mfr-for-treating-compression-and-pain/
https://trainingandrehabilitation.com/identify-treat-lumbar-plexus-compression-syndrome-lpcs/
https://trainingandrehabilitation.com/causes-hip-pain-how-to-fix/