Anti-GAD antibodies in HIGH titre are associated with stiff-person syndrome. Low titres are not and can be found in patients with endocrine problems such as diabetes mellitus.
Your symptoms are not suggestive of stiff person syndrome, so together with a low anti-GAD level, a diagnosis of stiff person syndrome is premature. The pain is more suggetive of a thoracic radiculopathy (pinched or inflamed nerve root in the back) which can occur from diabetes, Lyme disease, viral infections or a prolpased disc.
You should be check thoroughly for diabetes or glucose intolerance (pre-diabetes) with a formal glucose tolerance test if not done already
Good luck
Also, could the back pain be related to the endocrine problems?