How can I get concrete out of ground?
If they had a 32' play structure resting on top of them and they had 6" U brackets I'm guessing the footings are at least a foot in diameter? Maybe more? Keep in mind that even if you could dig the entire thing out of the ground it's going to weigh hundreds of pounds. Unless you have some heavy equipment to pull them out you're going to have some trouble moving them. There isn't any chance you can use them as a base for the shed, is there? If not, and you absolutely have to get them out, you might look into hiring an excavating contractor to do it. It's hard to make suggestions without seeing exactly what you're dealing with, but it sounds like an excessive amount of work to do by hand.