I am going to suggest you splint her leg or use a body sling,reason is b/c she has the use of the rest of leg,just not the lower half,it could be an injury/dislocation. It is worth a try as she cannot go on like this. If it is painful,remove splint. Use a body sling(hanging t shirt cut with two holes for legs,make sure feet touch ground)in the event it is a slipped Achilles tendon, There is a way to put tendon back into place,but b/c you do not know what is wrong and it has been left too long,it will be very painful. She is probably in pain,but b/c your vet has her on meds(is she on Prednisone) and i do not know what they are,i am hesitant to suggest pain medication. You could try an epsom salt soak,this is great for sprains.sore muscles. I am curious as to why your vet did not take any x rays of leg just in case it is an injury. You could also give her vitamin b-complex supplements which help with inflamed nerves.
It could also be symptoms of Marek's Disease as previously posted.