Thank you for the additional information.
The problems laying down not running and walking backwards is most often associated with joint or spinal pain. When we combine that with the fever the following things we need to consider are
1. Tick born illnesses such as lyme disease, rocky mountain spotted fever and Anaplasmosis. These illnesses can take months to show symptoms so even if you have not seen a tick on her we can not rule it out. The following link will go over all tick borne diseases in more detail LINK
2. Valley fever. I asked where you lived because we do not see this in all states however Tennessee is a state where we are seeing increased cases. Some dogs will only get the respiratory form while others develop what is called Disseminated valley fever and this is system wide effects the joints as well. This is a fungal infection so antibiotics would not cure it. Here is more on this LINK
3. Bacterial Meningitis. This would take in most cases a MRI or spinal tap to properly diagnose. LINK
I know you have already been to the vet but it sounds like they have missed something and additional testing needs to be done. I would discuss the following with your vet and try to get started on some testing. If this was my dog I would probably start with a tick panel which is a simple blood panel and go from there.
I hope this helps give you some direction. Please reply back if you have additional questions and I will be happy to continue. If all of your questions have been answered please take a few moments to provide a rating so that I am compensated by justanswer for assisting you today.