Combine the garlic and onion powder, thyme, oregano, sage, black pepper, salt, cayenne pepper, mustard powder, paprika, and celery seeds in a small bowl, and set aside.
Rinse and pat the chicken dry with paper towels. Generously coat the chicken with the spice mix, covering every inch.
Let it sit covered in the fridge for about an hour (or 24 hours for prepping it ahead) for the flavors to penetrate. Let your chicken come to room temperature before smoking.
When ready to smoke, preheat the smoker or pellet grill to 225°F (110°C) and double-check the hopper to make sure it has enough pellets. If using a charcoal grill, place the coals on one side to create a hot side, leaving the other side for indirect heat.
Clean and lightly oil the grates, and place the chicken directly on them.
Close the lid and smoke for 3-4 hours, or 45-60 minutes per pound. An instant-read thermometer should read an internal temperature of 165°F (75℃).
Remove the chicken from the smoker and let it rest for 15-20 minutes so the juices reabsorb into the meat and are evenly distributed.
Shred the chicken with two forks until the meat is shredded into bite-sized pieces.
Serve hot as a main dish or make sandwiches, tacos, or salads.