In a crock pot, combine the chicken, onions, celery stalk, garlic and broth.
Cover and cook on low for six to eight hours (or cook on high for four hours).
Remove the chicken from pot, reserve 1⁄2 cup broth and discard the rest.
Shred the chicken with two forks then return it to the slow cooker with the 1⁄2 cup broth and the hot sauce.
Set the temperature on high for an additional 30 minutes. Meanwhile, prepare the dressing.
In a small bowl, mash the blue cheese and yogurt together with a fork. Stir in the mayonnaise, lemon juice, vinegar and garlic powder until well blended. Season with salt and pepper. Refrigerate until ready to use.
To prepare the lettuce wraps, place 1⁄2 cup buffalo chicken in each leaf.