1. Prepare Cake: Grate zest from grapefruit to equal 3 Tbsp. Cut grapefruit in half, and squeeze juice to equal 10 Tbsp.
2. Preheat oven to 375°. Stir together melted butter, 1/2 cup grapefruit juice, and 1/2 cup water in a small bowl.
3. Stir together flour and next 4 ingredients in a large bowl. Stir in pecans, next 3 ingredients, 2 Tbsp. zest, and butter mixture until smooth. Pour batter into a greased (with shortening) and floured 13- x 9-inch pan.
4. Bake at 375° for 20 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool 1 hour.
5. Prepare Frosting: Beat cream cheese and softened butter at medium speed with an electric mixer 1 to 2 minutes or until creamy. Add vanilla and remaining 2 Tbsp. grapefruit juice and 1 Tbsp. zest, and beat until blended. Gradually add powdered sugar, beating at low speed until blended. Spread frosting over top of cooled cake.