This thread got me started thinking, and I found a couple of neat ideas. One is made to slide onto a set of forks, but looks like it could easily be adapted to work on a three point hitch. Looks pretty simple, so it might be worth building at home. The factory made version is rather expensive. It would eliminate having to get off the tractor seat, though. When you lift, the jaws tighten and bite into the tree, when you lower they should open and release it. Click
here.
A bit fancier idea I saw was a set of hydraulic jaws. It was one fixed jaw that could not move, and the other jaw was hinged on a hydraulic cylinder operated by a remote line. You could control how hard it squeezed, so it seems like that might work better for metal posts. Not sure how well the first one would grab a T post, unless you could hook it from the side. If you're just replacing one post and not tearing out the whole fence, you would have to grab it from the front or back.
I'm thinking I might try to build one of the ones like the grubber I linked to. Sure would be handy.