Prepare pineapple: Peel pineapple, reserving 3 to 4 strips of peel approximately 2 inches wide. Cut pineapple in half widthwise. Core both halves. Chop one half into ½-inch chunks. Slice remaining half into ¼-inch rounds for serving.
Mix spices: Combine kosher salt, dried oregano, cumin, garlic powder, and chipotle powder in a small bowl.
Season pork: Place pork shoulder chunks in the slow cooker. Sprinkle spice blend evenly over all pieces.
Add wet ingredients: Add pineapple chunks, entire can of chipotles in adobo, quartered onion, crushed garlic, chicken stock, and apple cider vinegar to the slow cooker. Stir gently.
Add pineapple peels: Place reserved pineapple peel strips cut-side down on top of everything in the slow cooker.
Cook: Cover and cook on HIGH for 5 to 5 ½ hours or LOW for 10 hours until pork shreds easily with a fork.
Shred: Remove and discard pineapple peel strips. Shred all pork directly in the slow cooker using two forks. Mix shredded meat into the braising liquid.
Serve: Warm corn tortillas. Serve al pastor with pineapple rounds, fresh cilantro, diced white onion, lime wedges, and salsa.
