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For the Guajillo Marinade, wipe outside of chilis with damp paper towel to remove any dust. Heat large deep skillet on medium-high heat. Toast chilis in hot pan until fragrant, about 10 seconds per side, working in batches as needed.
Place chilis in large heat-safe bowl. Add boiling water; cover. Soak chilis 30 minutes or until softened. Remove chilis with tongs or slotted spoon and transfer to blender container, discarding soaking liquid. Add remaining marinade ingredients; cover. Blend on high speed until smooth. Reserve 1/2 cup of the marinade; set aside.
For the Tacos, place pork in large resealable plastic bag or shallow baking dish. Add remaining marinade and turn to coat well. Refrigerate at least 4 hours or overnight. Remove pork from marinade. Discard any remaining marinade.
Preheat grill or gill pan on high heat. Brush pineapple with 1/4 cup of the reserved marinade. Grill 3 to 5 minutes or until lightly charred and softened, turning halfway through. Remove from grill. Once cool enough to handle, cut into bite-size pieces. Set aside.
Sprinkle pork lightly with additional kosher salt, if desired. Grill pork over high heat until lightly charred and cooked through, about 2 to 4 minutes, basting with remaining 1/4 cup marinade. Let rest 5 minutes. Slice into thin strips.
Transfer pork and pineapple to serving bowl, tossing to mix. Serve mixture in warmed tortillas, topped with onion, and cilantro. Garnish with lime wedges.