Preheat oven to 350 degrees F. Grease and flour a 10-inch fluted tube pan; set aside.
In a small bowl combine milk, the 2 teaspoons grapefruit peel, the 2 teaspoons lime peel, the 2 teaspoons orange peel, and the grapefruit juice. Mix well.
In a large bowl combine sugar and the 1 1/4 cups butter. Beat with an electric mixer on medium speed until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in vanilla.
In a medium bowl combine flour, baking powder, baking soda, and salt. Alternately add flour mixture and milk mixture to butter mixture, beating just until moistened after each addition.
Spread batter into prepared pan. Bake for 40 to 45 minutes or until a toothpick inserted near the center of the cake comes out clean. Cool in pan on a wire rack for 10 minutes. Remove cake from pan; cool completely on wire rack. (To serve today, omit Step 6 and continue as directed in Step 7.)
Wrap cooled cake in plastic wrap; overwrap tightly with foil. Freeze for up to 1 month. Thaw overnight in the refrigerator before serving.
To serve, in a small bowl combine the 2 tablespoons melted butter and 1 tablespoon of the orange juice. Add powdered sugar; beat until smooth. If necessary, add enough of the remaining 1 tablespoon orange juice to make drizzling consistency. Drizzle over cake. If desired, sprinkle with additional grated peel.