It sounds like it needs an adjustment if it is ghost shifting. Should be about $10 at your lbs. They should also check the hanger, if that is tweeked, no matter how dialed you set up the derailleur, it's not going to be perfect. If it is not shifting at all, that might be a problem with your shifter. Over the years, the pawls in the shifter become brittle, and the shift just won't catch.
If you were to replace the XT w/ X.9, you'll have to replace the shifters as well.
In short, take it your lbs, they'll be able to tell you if it needs an adjustment, or if it time for some new components.