Ouch... that does not sound good. It sounds like someone severely overtightened the box in the past and either a) the worm and sector have worn on center creating a low spot or, b) the worm is slightly bent. Neither sounds good. I don't know what a replacement worm and sector shaft would run but can't be cheap. I wonder what the smart shopper price is.
I don't know how much you would be in for a rebuilt box, but I think that you should be able to pick up a good used unit and do the reseal yourself with a factory seal kit for a couple hundred bucks.
I don't know how much you would be in for a rebuilt box, but I think that you should be able to pick up a good used unit and do the reseal yourself with a factory seal kit for a couple hundred bucks.