Place pork shoulder in a slow cooker and top with 1 cup of the diced pineapple and the diced onion; set aside.
Remove seeds and stems from dried chilis and place in a large bowl. Cover with boiling water and let steep to soften, about 15 minutes.
Place softened chilis along with 1 cup of the reserved steeping liquid in a large food processor. Add the remaining 1 cup diced pineapple, quartered onion, lime juice, achiote paste, vinegar, garlic, cumin, oregano, salt and pepper. Puree on high for 60 seconds.
Drain mixture over a fine mesh strainer placed directly on top of the pork shoulder in the slow cooker insert. Use a rubber spatula to press out all of the liquid and discard what's left in the strainer.
Cook pork on high for 4 hours or low for 8 hours.
To serve, shred pork with two forks discarding any large pieces of fat. Serve in warmed corn tortillas with cilantro, minced onion, minced pineapple and any other toppings of choice.