Preheat oven to 375°. Crush pretzels into crumbs using a food processor or mallet. Add sugar, butter and 2 tablespoons of limeade concentrate. Mix well. Spoon mixture into an ungreased 9-inch pie plate. Press mixture firmly into bottom and up sides of pan. Bake about 5 minutes. Freeze for 10 minutes. Combine ice cream, remaining limeade concentrate, tequila and liqueur. Mix until just blended. Spoon ice cream mixture into cooled pie shell, and sprinkle with lime zest. Freeze for 2 hours before serving.