Add the grapes and the lime zest to a blender; blend on high speed for 2 minutes. Add 1 1/2 cups of the soda and puree for another minute. Pour into a 9-by-13-inch baking pan and freeze for approximately 3 hours, or until frozen. Using a fork, scrape and stir the mixture, forming little shards of frozen grape puree.
To serve, scoop the slushie into small juice glasses, top with 1/4 cup lemon-lime soda, and garnish with a red grape skewer. Serve with a spoon and a straw.
To make the skewers, simply place 3 to 4 red grapes on each toothpick or skewer, and freeze on a rimmed baking sheet.