Hold on a second.

Before you take major repair or replacement steps do the simplest thing first...put some lube on all the pivot (moving) points of the caliper then operate the brake several times and see it that frees it up. If not then try loosening the pivot bolt head while holding the rear mounting nut in a fixed position, just loosen about 1/16ths of an inch then test again, if it works better but still a bit stiff then loosen the nut again about 1/16ths of an inch and retest.

Here's a site that shows detail pics on how to adjust the caliper: http://www.parktool.com/repair/readhowto.asp?id=22