I’m not a physio – so seek advice from a fully qualified physician to asses the extent of the damage before commencing any rehab exercises, and to try to work out the exact cause of the problem – possibility could be neural problem if it is a reccuring injury although sounds like your doing the right thng by trying to correct your pelvis problem. Scar tissue build up from previous tears may also be causing a “weak link” in your hamstring – use of Deep tissue massage to break this down down could help. Foam rollers before and after exercise may help reduce tension in the area.
I have seen a large decrease in the occurance of hamstring tears and strains by introducing Nodric Hamstring exercises into a conditioning program. Essentially a eccentric hamstring exercise. If done correctly can really help strenghten the hamstrings to help prevent future problems. There is a short vid on you tube http://www.youtube.com/watch?v=16Uf7GSjw1w Key is keeping alignment from head through shoulders down to hips and into knees all the way through the movement. Break the alignment at hips and the focus will be shifted off the hamstrings.
Pilates/yoga may also be worth looking into for flexibilty increases and core strengthening, provided you are cleared to do so by chiro/physio.
Hope this helps a bit. Will be interested to see if others have had success with this exercise?