Bring a large pot of water to a boil over high heat. Set up a medium bowl of ice water. Drop the shrimp into the boiling water and cook, 2-3 minutes, until shrimp turn pink and curl up.
Remove shrimp and immediately plunge into the ice water. Allow them to cool. Pat the shrimp dry with paper towels and chop shrimp bite sized pieces. Set aside.
In a large mixing bowl, combine the dressing ingredients: Whisk the mayonnaise, Greek yogurt, lemon juice, vinegar and salt together. Stir in the tarragon and parsley.
Fold the shrimp, chopped celery, diced onion and diced avocados into the mayonnaise mixture. Sprinkle with crushed black pepper, if desired. Serve chilled.