A couple thoughts:
1) Most of the speculation on Locke's returning paralysis focuses on why the Island might be punishing him (lack of faith, not following directions, etc.), but what if the Island was actually protecting him? His legs being useless, he was forced to stay on the ground for the beechcraft expedition, requiring Boone to climb up and explore the plane instead. Once the plane had crashed to the ground and was safely accessible, Locke was able to start walking again and carried Boone back to the caves. As alpha male, Locke undoubtedly would have insisted on going up and exploring the plane so he could find what the Island wanted him to use on the hatch, so the Island physically stopped him and forced Boone to go instead.
2) Although the plane had always been unstable, notice that it began to aggressively slide when Boone attempted to communicate with the outside world. Another instance of bad things happening for those trying to facilitate escape.