Your low back pain could be stemming from toxic overload in the kidneys. If there isn't enough salt in the body, the kidneys will not release the toxins into the urine. The kidneys actually need salt to release toxins!
Having said that, you want a good quality salt, (not tablesalt) for this.
Personally, every time I did a salt-water cleanse, with this kind of salt found at my health food store, I felt LOADS better.
I did 4 or 5 ONE DAY salt-water cleanses during the course of my 32 day fast.