Preheat the oven to 325 degrees F (165 degrees C). Place chicken in a small baking dish. Reserve 1 tablespoon ranch seasoning and sprinkle remaining seasoning over chicken. Pour barbecue sauce over chicken, cover with foil.
Bake in the preheated oven until chicken is done and shreds easily with a fork, 50 to 60 minutes.
Increase oven to 400 degrees F (200 degrees C). Working with 2 pans or in batches with 1 pan, lightly coat a rimmed baking sheet with avocado oil.
Bake tortillas in the oven for 4 minutes.
Meanwhile, shred chicken and return to the pan with juices; set aside. Top tortillas evenly with shredded chicken and cheese.
Bake tacos until crisp, 8 to 10 minutes. Fold tortillas in half to form a taco and top with shredded lettuce and pickles.
Stir together remaining ranch seasoning with sour cream, mayonnaise, and pickle brine. Serve sauce with tacos.
