I've had the same symptoms (including the agonizing one-hour car ride symptom) for over a year. Thinking that it was a chronic muscle tear, I went to massage therapists, physical therapists, and the founder of Active Release Therapy. Eventually I was referred to a physiatrist (M.D.) who thought that it was probably a stress fracture. I was skeptical, but got a bone scan anyway. Based on the bone scan, the diagnosis was a stress fracture or avulsion fracture of the ischium. The main "treatment," of course, is rest.
I think that there are a lot of problems that can arise in that area and produce similar symptoms. In addition to fractures and chronic hamstring tears, there can be neurological problems in the lower back that will manifest themselves in the area of the buttocks and hamstrings insertion. Compartment syndrome can also show up in that area.
I've received a lot of well-meaning advice from many people who have experienced similar symptoms, but may have had diffent injuries. You may have to try a number of different specialists and treatments before you hit upon the solution. Depending on your budgetary constraints, I would consider getting a bone scan or MRI. More conservative physical therapy (massage, ART, stretching, strengthening, and so on) may work for most soft-tissue injuries, but could aggravate other injuries.
I'd be interested in hearing from others on this topic. In any event, let us know how things go for you.