Pain is major cause of shaking in dogs. These pain conditions in dogs can result from disc injury, tissue damage, tissue inflammation, muscular and skeletal problems, heat and cold exposure, traumatic injury, and some other diseases. You should also check for dog fleas because fleas and skin parasites can also cause shivering in dogs. If your dog is declared fit by your vet then it will be better to get second opinion and if second opinion is also in your favor then no need to be worried. In my opinion, difficulty in walking always has some underlying cause.
