I need a creative solution for replacing an outlet box.?

I'm remodeling my bathroom and after setting up the vanity/countertop I was pissed to see that the countertop goes above the electrical outlet by about 1/2 inches. I decided that I'd just move the outlet up about an inch or so and patch up the bit of exposed hole in the drywall. The old outlet box was nailed into a stud and after eventually tearing it out I'm clueless as to how I'd replace it with another nailed in one as there is no way to hammer it in without making a huge hole in the dry wall. I tried one of the outlet boxes with the tabs that you tighten with screws but because of the open space at the bottom of the drywall the bottom tab has nothing to tighten against. What's the simplest and most effective solution?