In a small bowl, whisk together the mayonnaise, yogurt, 1 tablespoon lemon juice, and 1 tablespoon sugar until smooth to make the dressing; set aside.
On a cutting board, core and chop the apples into cubes and halved the grapes; place the apples in a large bowl, add 1 tablespoon lemon juice, and toss to coat to prevent browning.
Add the cubed cooked chicken, diced celery, chopped nuts (1 cup), halved grapes, and raisins to the bowl with the apples.
Pour the prepared dressing over the salad ingredients and gently stir until everything is evenly coated.
Cover the bowl with plastic wrap and chill until ready to serve.
Garnish with the whole walnuts or pecans before serving.