If this has not progressed to her front end then its likely a neurological issue impacting rear leg use. Might be FCE
http://www.marvistavet.com/html/body_fibrocartilaginous_embolism.html
A tumor, an injury (hit by car perhaps) or could be due to disc issues which you can read about here
http://www.gcvs.com/surgery/interver.htm
She might recover with surgery depending on the problem.
She might do great with a cart
http://www.dogkarts.com/
She might improve depending on the issue with acupuncture, swim therapy or other holistic care if you work with a holistic vet. This site may help you find one to consult
http://www.holisticvetlist.com/#search
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics