I have had x-rays, an mri, nerve tests, and the doctors say they are all negative, but I still have a pain in my lower back, that shoots down my leg?

Whoops. The online address for the aao is academyofosteopathy.Org, not aao.Com sorry.
Misalignment. Many misalignments are biomechanical (how your body lines up and works - or doesn't) and may not show up on imaging. They can be caused by old injuries, compensations to injuries or sugeries, deconditioning or by life. Now that you know it's not something scary, find a good do who does manipulation (biodo.Com, aao.Com, cranialacademy.Com). Once alignment is better, consider adding phys therapy.
Further work up. There are other potential causes of neurological leg pain, some of which are dynamic and so they do not show up on nerve testing; piriformis stndrome is one exam. There are non- neurological causes as well including vascular disease. A detailed history, exam, and appropriate studies may indicate the cause.
See spine specialist. It is possible to have these pains despite normal studies. A non-surgical spine specialist might be able to localize your pain generator. You can have nerve irritation from the spine that was not apparent on the MRI and emg's are frequently negative. In these cases diagnostic injections with anesthetic only can be used to help identify the source of your pain.