1. Peel and section grapefruits; reserve juice that drains from grapefruit. Sprinkle avocado slices with reserved grapefruit juice. Arrange grapefruit sections and avocado slices pinwheel fashion on a lettuce-lined serving platter. 2. Combine remaining ingredients in a small mixing bowl; mix lightly with a wire whisk. Spoon dressing over salad.
