In a large bowl, combine the pork shoulder slices, pineapple juice, white vinegar, orange juice, adobo sauce, chili powder, cumin, oregano, garlic powder, onion powder, salt, black pepper, and cayenne pepper (if using). Mix well to coat the pork evenly. Cover and refrigerate for at least 2 hours, preferably overnight for maximum flavor.
Preheat your grill or stovetop grill pan over medium-high heat.
Remove the marinated pork from the refrigerator and let it sit at room temperature for about 15 minutes.
Grill the pork slices for about 4-5 minutes on each side, or until they are nicely charred and cooked through. Add the pineapple rings to the grill during the last few minutes of cooking, turning until caramelized.
Once cooked, remove the pork and pineapple from the grill. Let the pork rest for a few minutes before slicing it into small pieces.
Warm the corn tortillas on the grill for about 30 seconds on each side until soft and pliable.
To assemble the tacos, place a generous portion of the grilled pork on each tortilla, top with diced onion and chopped cilantro, and add a slice of grilled pineapple if desired. Serve with lime wedges on the side.