In a blender, combine pineapple juice, vinegar, achiote paste, guajillo chili peppers, garlic powder, oregano, cumin, salt and pepper. Blend until smooth.
Slice the pork shoulder into thin slices, then transfer to a large dish or bowl and pour the marinade over the pork slices. Toss to make sure they are coated on all sides. Refrigerate for at least 2 hours or up to 3 days.
Preheat the oven to 350 degrees. Line a 9x13 inch baking pan with foil. Load sliced pork and pineapple onto vertical skewers. Ideally, a specialized rack is easiest. Alternatively, buy a second pineapple and peel. Line a two baking sheets with foil. Cut into four large, thick rounds and place two rounds each into two loaf pans. Put two skewers into each pineapple round and alternate 2 to 3 slices of pork with a slice of pineapple. Place loaf pans on baking sheets.
Bake for about 1 1/2 hours, or until the pork is slightly charred on the outside and deep red. Every so often, check to pans and drain any juices with a turkey baster. Rest the meat for about 10 minutes, then carve off thin slices of pork and roasted pineapple.
To assemble, place some pork and pineapple on the tortillas, top with onion, cilantro, tomatillo chipotle sauce and avocado. If desired, serve with lime wedges.