In a large glass measuring cup or mixing bowl, combine the pineapple juice, brown sugar, ketchup, soy sauce, garlic, Worcestershire sauce, and seasonings. Whisk until all ingredients are incorporated; set aside.
Heat 2 Tablespoons of butter in a large skillet over medium high heat. Add the chopped onion and sauté until onion begins to brown.
Stir in 1/4 cup flour. Add pineapple juice mixture to the skillet with the butter, onion and flour. Bring to a boil.
Boil, stirring, 10-15 minutes or until sauce is thick.
Place the chicken in the crockpot and pour the sauce it. Cook on low heat 8 hours. Shred using 2 large forks.
Serve on sandwich rolls, over rice, or on top of your favorite salads. *If sauce is not as thick as you'd like it to be, you can prop open the lid and let some additional liquid evaporate before shredding and serving. Note: sauce will thicken further as it cools.