In a mixing bowl, combine mayonnaise, sour cream, lemon juice, garlic powder, tarragon and salt. Set aside.
Roughly chop the dried cranberries and finely chop the walnuts.
Combine chicken, onion, celery, cranberries and walnuts in a large bowl.
Add the mayonnaise mixture and toss well to combine. Serve immediately or refrigerate until ready to eat.