Your dog might have a nerve, hip or pelvis problem causing the weakness. There are several things I have used when there is rear-end weakness. I would try Promaxol to help healing to begin. Promaxol would be a good choice for your dog. It contains the stronger LM potencies. After improvement is seen then you can switch over to Arthro-IonX. In addition it is helpful to give Canine Probiotics with each meal, Omega 3,6,9, MSM, and vitamin C daily. (We carry the first two and the other two you can get at a health food store.) To aid the nerves I would give a Choline/ B vitamin supplement such as Geri-Form. The oligotherapeutic mineral base found in the IONX products helps the body to better utilize the other nutritional substances given for a synergistic response and faster healing. Also be sure to give sufficient fresh water that is free of chlorine and fluoride. If you can gradually phase into a grain free diet that can be helpful too. The other thing that I find beneficial is to have a certified veterinary chiropractor check for spinal alignment and make any corrections as necessary. Acupuncture can be valuable as well.
Please let us know how you progress or if you have any other questions.