Start by spraying crock-pot with Pam or non-stick spray to prevent sticking.
In a large bowl, whisk together the spice rub ingredients.
Start by washing the pineapple and cutting the top and bottom off to create a flat base. Stand the pineapple up on one end and halve the pineapple lengthwise. Core (throwing out cores afterwards) and dice two pineapple halves into ½-inch pieces. Using a large knife, slice the remaining skins into 2-inch-wide strips and set aside.
Cut the pork butt into two-inch chunks, adding to the spice mix and tossing to coat evenly, then transfer to the slow cooker. Add the diced pineapple, onion, and garlic. Add the chicken stock and white vinegar, and top with the reserved pineapple peels, cut/fruit-side down.
Turn the slow cooker to HIGH and cook until the meat falls apart with a fork, 5 to 5½ hours.
To serve: In a separate bowl, pull apart the pork with a fork and transfer to a large platter with the pineapple and onions. Serve with tortillas or brown rice, cilantro, or lime wedges.