Puree onion, chipotles, garlic, vinegar, chili powder, cumin, and diced pineapple with 1 teaspoon salt in a food processor or blender, pulsing for about 1 minute or until smooth.
Stir beer into the mixture.
Put the pork in the slow cooker.
Pour pineapple and beer mixture over pork.
Cover pot and cook on high heat for 4 hours or low for 8 hours.
Once the pork is cooked, shred with a fork.
Serve pork with tortillas, extra pineapple, cilantro, and diced red onion, or any preferred sides.