Place the chicken tenders and chicken stock in a large pot and bring to a gentle simmer over medium heat; poach for 15–20 minutes until the chicken is fully cooked and reaches 165°F (74°C).
Remove the chicken from the stock and let cool slightly, then shred the chicken using a stand mixer with a paddle attachment or by pulling apart with two forks.
In a medium bowl, combine the mayonnaise, finely minced celery, dry ranch dressing mix, salt, and ground black pepper; stir until smooth and evenly mixed.
Add the shredded chicken to the dressing and mix thoroughly until coated.
Fold in the red seedless grapes, chopped Fuji apple, and chopped pecans until evenly distributed.
Serve the chicken salad chilled on a bed of lettuce or in sandwiches.
