Place the chicken tenders and chicken stock in a large pot and bring to a gentle simmer over medium heat.
Poach the chicken for about 15 to 20 minutes, or until fully cooked and the internal temperature reaches 165°F (74°C).
Remove the chicken from the stock and let cool slightly, then shred using a stand mixer with the paddle attachment or two forks.
In a medium bowl, combine the mayonnaise, minced celery, dry ranch dressing mix, salt, and black pepper; stir until smooth.
Add the shredded chicken to the dressing and mix until evenly coated.
Fold in the halved grapes, chopped Fuji apple, and chopped pecans until incorporated.
Chill for a couple of hours if possible, then serve on lettuce or as a sandwich filling.