I presently have a foster GSD/Lab. He was tied and beaten for 4 years and always lived outdoors. I too faced the same problem but initially it was just to walk period. He would 'crawl' on his belly to get from point A to point B. I had to carry this big lug to the car.
When putting on the leash I was faced with the alligator spin, followed by screaming and excessive lashing and then panting. Basically, I just let him tire himself out and then would try to walk. No go.
That same day I decided to use my little helper, that being a min pin. Julia has helped me re-hab about 5 dogs to date and has always been my 'ticket' to success. Once I had her outside with him, things took a dramatic change. Julia would walk and bark, and he would take another step forward. It took about 15 minutes and though he was not completely up right - he was moving. I would stop when he would and when Julia wanted to move forward - he took her lead. We then worked on 15 stairs - which is now a no brainer.
If you can use another dog to assist you - it really is the best method in my opinion. Dogs mimic one another and their trust is alot more effective. (please no one come down on me for saying this - but it is what I experience).
Try another confident dog to help you accomplish the walk. My foster has been with me for 3 weeks now and he is not the same dog at all. All my neighbours cannot believe the difference, and I must say that Julia played a big part in 50% of the training.