In a large mixing bowl, combine the cooked chicken, celery, red grapes, and chopped pecans. Mix well to combine all the ingredients evenly.
In a separate bowl, whisk together the mayonnaise, sour cream, Dijon mustard, garlic powder, onion powder, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is well coated.
Taste and adjust seasoning if necessary, adding more salt or pepper as desired.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld.
Serve the chicken salad on a bed of lettuce leaves or as a sandwich filling in your choice of bread.