I believe John Locke has been dead since the crash of Flight 815. There are logical (island) reasons such as radiation why Rose was in cancer remission and fertility was messed with, but there really isn't an explanation as to why Locke's legs worked again. There was something in John's makeup that the island needed and therefore made him a spirit/Locke combination vessel. He was aware of the island's special properties almost immediately upon awakening. His knowledge of when it was going to rain and non-fear of the smoke monster suggest at least part of him was of the island. Since dead is dead, when Locke returned to the island, he needed a new co-habitant for his body to return to life. I think that is why Locke seems different now. He is inhabited by a different entity. Theory by WNGYPSY
