Bring 2 quarts water, 1 tablespoon lemon juice, and 2 teaspoons salt to a boil in a 3-quart saucepan, then poach shrimp at a bare simmer, uncovered, until just cooked through, about 3 minutes. Transfer with a slotted spoon to a bowl of ice and cold water to stop cooking. Let shrimp chill in water 2 minutes, then drain and pat dry. Whisk together tarragon, mustard, pepper, and remaining 2 tablespoons lemon juice and 1/2 teaspoon salt in a serving bowl. Add oil in a slow stream, whisking until emulsified. Quarter avocados lengthwise, then pit, peel, and cut into bite-size pieces. Add shrimp, avocados, and endive to dressing and toss.