In small bowl, whisk together Chipotle Sour Cream Sauce ingredients, cover, chill. (*Tip: I always add a can of chipotles in adobo sauce into a mini chopper and puree them with the sauce. Then add the mixture to a small freezer baggie and freeze. When you need some, just break off a piece.)
In medium-sized bowl or jar with a lid, add Seasoning Mix ingredients, whisk or shake to mix completely.
Cut chicken in half, (lengthways, crossways or into pieces), remove skin and fat, and place into large oval crock pot/slow cooker bone sides up. Sprinkle with a little less than half of Seasoning Mix, turn over and sprinkle meaty sides with rest.
Whisk together wet ingredients, pour over chicken. Sprinkle with finely chopped onion and crushed red pepper flakes.
Place on lid and cook on HIGH for about 5 - 6 hours until meat is done and easily pulls away from the bone, *turning meat over and nestling down into broth halfway through cooking time.
Remove chicken halves to a baking sheet, remove bones, and pull chicken with 2 forks. (*Check through the meat with your fingers to make there aren't any small bones left behind.) Place chicken meat back into cooker and stir.
Now. There is ONE last step! Pour everything into a small stock pot or very large saucepan, bring to a boil, turn down heat until it's just at a steady low boil, and cook 30 more minutes, stirring every so often, to thicken sauce. This also incorporates the sauce into the chicken pieces. Don't skip this step, nor try to just finish cooking it in the crock pot. It doesn't work.
To Assemble: With tongs, pile pulled chicken into warmed tortillas or taco shells. Top with lettuce, tomato, cheese, guacamole and Chipotle Sour Cream Sauce. Serve with extra hot sauce or taco sauce for those who want a little extra heat. And enjoy!
5 - 6 boneless skinless chicken breasts can be subbed for the whole chicken if you prefer.