I've been re-watching the show from the beginning and I noticed something interesting.
In episode 1 Granny Smith could barely walk. Then over the course of the four seasons she becomes less and less hindered. And then season 4 has Leap of Faith revealing she still quite athletically capable.
Was it a mental thing the whole time? Did they manage to get her a new hip?
well in the ticket master episode (I think :s) Aj said that she needed to buy granny a new hip, so im guessing it was more of a healing then they found the money to replace it. Just a guess
probaly both
She also says they've had an Apple Family Reunion every hundred or so years. And she's in every one. So either Equestrian ponies live far longer than humans, or Granny Smith has access to the golden Apples of Youth from Norse Mythology, and they just are taking a while to take effect.
It was actually every hundred moons, not years, which if they have any sort of lunar cycle like ours it would be roughly 7 to 8 years between every major reunion. I'm sure someone will bring up the reunion in episode one, but I figure it's something like what my family used to do where we would have a small get-together every year that we still called a reunion, but every few years there'd be a big one that everyone tried to get to that would also be called a reunion. The latter matching the type of reunion granny smith was talking about while the former would be closer to the one in episode one.
However, the reunion thing has been argued left and right and is a bit off-topic.
I do find it odd how her mobility has improved over the series. Part of it could be that they were able to pay off the medical help she needed which would explain why it hasn't been mentioned in a while. It could also be that it really was a mental block especially after the episode Leap of Faith where she just needed a boost in self-confidence. Of course it could also just be the writers disregarding her mobility issues from earlier in the series so that they could do more with the character than just sticking her with cliches.
Of course it could be all of the above as well or something else entirely too.