Use a sawsall with a metal cutting blade. Put the blade between the box and the stud and simply cut the nails. Suggest you replace the old metal boxes with heavy blue plastic remodel boxes.

To clarify...I will need to cut up into the sheetrock to cut the nail (similar on the bottom nail), so there is no real clean way to avoid drywall damage. The nail is probably 2 to 2.5 inches above the top of the box on the flange.

I've done this many times with minimal wall damage. First, take a sheet rock saw and make a special, vertical cut all the way up the side of the box away from the stud; what you want to do is cut at a 45 degree angle so that the face of the sheetrock is not affected, but the backside is relieved. The reason you do this is to give yourself some room for the next step. Get a pry bar and insert it between the side of the box and the stud that it's nailed to; then pry so you get some clearance between the side of the box and the stud. the box will swing into the cavity and clear the sheetrock where you've relieved it. If the nails are short, you may actually be able to get the box pried far enough so that it comes free. If it doesn't, take a hack saw blade and wrap one end in tape to protect your hand, and then carefully cut through the nails at an angle so that you're not cutting the sheetrock. If you end up damaging the sheetrock, you have two other options. First, get an oversize wall plate that will cover the damage. Or, enlarge the hole sideways and make it big enough for a double receptacle box. One of these solutions should work for you.

This may be a slightly different tack that's worked for me in the past--

In the OP case, I'm visualizing a box with fairly long mounting straps attached. (That accounts for the nails being 2 or 2 1/2 inches above and below the box.)

Those straps...or ears...or mounting brackets (whatever you want to call 'em) are usually only spot-welded to the box at a couple of places. If you take a Sawzall (or hacksaw blade) and cut between the box and the strap, you should be able to separate them. (It might be possible to do this with a thin cold chisel as well, though that'd be messier.)

Once the strap is disconnected from the box, the box should be easy to remove, leaving you with a big hole to work out the strap.

If all else fails, take the Sawzall and cut through the strap at top and bottom of the outlet hole (being careful not to take too much out of the stud!). In this case, the strap segments will remain buried in the wall, but you should be able to ignore them when installing a replacement old work box, even if you have to bend them up and out of the way.

I want to thank terribletom for the advice. I faced the issue, bought a reciprocating saw and did the job - no problem, easy as pie. Thanks again!