Cut peels, including pith, from grapefruit; discard. Working over a bowl to catch any juices, cut segments from membranes, letting them fall into bowl. Squeeze the remaining juice from membranes into bowl.
Whisk together vinegar, honey, sour cream, salt, and 3 tablespoons grapefruit juice in a small bowl. Season with pepper. While whisking, add oil in a slow steady stream; whisk until emulsified.
Halve endives lengthwise, and core; cut leaves into 1-inch-thick pieces. Toss with three-quarters of the dressing. Cut avocado into 1/4-inch-thick slices. Divide endive and avocado among four plates. Drizzle each serving with one-quarter of the remaining dressing. Divide grapefruit segments among plates.