In a large mixing bowl, add the shredded rotisserie chicken.
Add the halved red and green grapes, celery, and walnuts to the bowl.
In a small bowl, whisk together the mayonnaise, sour cream, mustard, garlic powder, salt, pepper, and lemon juice until smooth and creamy.
Pour the dressing over the chicken mixture. Gently stir until everything is evenly coated and well mixed.
Cover and chill for at least 30 minutes before serving to let the flavors meld.
Give it a final stir before serving, and taste to adjust salt or pepper if needed.