A good musculoskeletal evaluation can help determine why the adjustments aren't holding. It is sometimes necessary to provide outside stability in the form of a special belt which helps hold the pelvic joints together once they are in good alignment. This would be worn daily for several weeks. There would also be specific exercises prescribed to help strengthen the stabilizing muscles of the back and pelvis.

Posture, body mechanics and positioning need to be reviewed; problems in anyone of those areas can be causing the adjustments to not hold. I would recommend a break from running or jogging at the present time, and continue your cardio fitness routine without any weight-bearing, high-impact activities. If you can continue the stationary or regular cycling without pain, go ahead.

As for the crunches, you may be doing these incorrectly and creating more of a problem. Your abdominal muscles may not be contributing to the stability of your low back and pelvis the way they should be. You may want to see a physical therapist that specializes in women's health, particularly prenatal and postpartum.

An excellent book that covers a lot of this is How to Raise Children Without Breaking Your Back by Hollis Herman and Alex Pirie. You can contact the American Physical Therapy Association's Section on Women's Health for assistance in locating a physical therapist specialist in your area.