Robyn's guess is far more likely than this one, but there are Cuban Tree frogs in the gulf south that are similar in description to that. They're mostly in FL, but they've been found in states further up. Their color can vary, but the yellow and black on the legs while being gray, does fit. (At least the ones I had on my porch had yellow under their legs with small/light black spots or stripes.) Most of their belly is usually white though, so it's just a guess, and makes the gray tree frog more likely.
http://www.wec.ufl.edu/extension/frogs/ost...entrionalis.htm