As I'm sure you know, this behavior is not normal. However, labs are prone to a condition called degenerative myelopathy which can also cause problems with the rear legs and paralyses. You can read about this here:http://www.petplace.com/dogs/degenerative-myelopathy/page1.aspxYou can read about testing here:http://www.mzjf.info/test.html
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In a lot of cases there is no pain, but complications due to the condition can cause pain. They do have DNA testing available and it sounds like you have had other testing done (MRI) that would rule out other conditions. So the first thing I would do is go ahead and have a DNA test done to check for DM. Even though it is not totally accurate to rule out DM, it at least helps determine if it is indeed DM. http://www.animalgenetics.us/ABIUKNew/horse-dna/canine/DM.htmlhttp://www.offa.org/dnatesting/dm.htmlI would suggest a raised platform for the food and water bowls or even a water bottle much like guinea pigs use might be a good choice. This would allow him to eat and drink without having to lower his head. I would get a light weight harness for him and use that rather than any type of neck collar. If he has an issue in the cervical (neck) area of the spine, you do not want to have any undue pressure exerted on it including petting which might explain the dog not being open to touching. I might also conside a CT scan or myelogram looking at the neck area. I really feel this is health based more than behavioral in nature and hope you investigate that avenue further. I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .