I had a similar problem with my 4.5.

Fortunatley, putting them in a vise and using the old pads as a buffer between the piston and vice jaws, I was able to press the pistons into the calipers (be careful when doing this, brake fluid will shoot out from the inlet hole!). So far I've not seen any adverse affects of doing this. I WAS planning on replacing the calipers, since I figured this meant that they'd go bad sooner or later, but I haven't lost a drop of fluid nor have my pads been dragging. Your luck and results may vary, depending on how rusty the pistons are.

If you need to do this though, you can be sure that the old rubber seals are beyond shot and that's how water got into them in the first place, so buy some rebuild kits!