In a large bowl, combine the chicken, celery, green onions and grapes.
In a separate small bowl, make the dressing by combining the mayonnaise, sour cream, brown sugar, lemon juice, salt, pepper and cayenne. Stir until combined, then taste and add more salt, pepper or cayenne, if you'd like.
Pour all but ¼ cup of the dressing on top of the salad. Sprinkle on the dill. Stir until all ingredients are coated in the dressing. Take a look and see if you think it has enough dressing for you; feel free to add more or the rest of it.
Cover the salad bowl with plastic wrap and chill the salad for at least 4 hours or up to 24 hours before serving, to let the flavors meld.