Blend the sauce. In a high-powered blender, combine the pineapple chunks, pineapple juice, chipotle chiles, adobo sauce, garlic, apple cider vinegar, tomato paste, honey, chili powder, smoked paprika, cumin, oregano, salt, and black pepper. Blend until completely smooth.
Add to the crock pot. Place the chicken breasts into the bottom of a 6-quart slow cooker. Pour the blended al pastor sauce over the chicken, making sure each piece is well coated.
Slow cook. Cover and cook on LOW for 5–6 hours or HIGH for 3–4 hours, until the chicken is cooked through and easily shreds with two forks.
Shred the chicken. Transfer the chicken to a cutting board or large bowl and shred using two forks. Return the shredded chicken to the crockpot and stir it into the flavorful sauce so every bite is coated.
Broil in the oven. For the best texture, spread the shredded chicken onto a baking sheet in an even layer. Broil for 3–5 minutes until the edges become slightly caramelized and lightly crispy. This extra step mimics the crispy edges of traditional al pastor cooked on a vertical spit.
Assemble the tacos. Warm the tortillas and fill each one with the shredded chicken. Top with fresh pineapple, diced onion, avocado, cilantro, cotija cheese, and a generous squeeze of fresh lime juice.