Salad: in a large bowl combine grapefruit, oranges, shrimp and red onion and set aside. Arrange lettuce on individual serving plates and spoon shrimp salad mixture onto lettuce. Just before serving, slice avocados and arrange on salad plate. Dressing: add first four ingredients to a blender or small mixing bowl. Slowly add the vinegar and blend at medium speed. Add the rest of the vinegar and oil, beating until creamy. Add celery seed and mix until blended. Original qty makes 3 cups. To serve: generously drizzle dressing on each salad just before serving. Per serving: 321.5 Cal; 27.7 G protein; 29.9 G carb; 275.8 Mg sodium; 5.5 G fiber; 11.6 G fat; 1.7 G sat fat; 221. 0 Mg chol; 96.1 Fat cal (29.9%);