In large bowl, combine Shrimp, grapefruit, oranges and red onion. Set aside. Arrange lettuce on individual serving plates. Spoon Shrimp salad mixture onto lettuce. Just before serving, slice avocados and arrange on salad plate. Generously drizzle dressing on each salad before serving. Dressing Preparation Add first four ingredients to blender or small mixing bowl. Slowly add part of vinegar, blend or mixing at medium speed. Add the rest of vinegar and oil, beating until creamy. Add celery seed.