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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
I'm sorry for the long wait.
It sounds like you are seeing a true neurologic disease. This is typically due to something pressing on the spinal cord and nerve roots. This could be a disc herniation, a tumor, a cyst, a stroke or even an infection like meningitis.
The only way to definitively diagnose the problem, is with an MRI of the spine. Given her age and breed, I would look into a consultation with a veterinary neurologist. They have the equipment to diagnose and then treat. Treatment could be medical if there is still good neurologic function to the back legs. It could also be surgical to remove a compressive disc that is hitting the spinal cord.
Here is a nice write up on disc disease in dogs:
I hope this helps, let me know if you have any other questions.