Cut the pork shoulder into 4 pieces, then place in a large bowl.
Blend the onion, pineapple, oregano, garlic, orange juice, chili powder, sea salt, and guajillo peppers until smooth.
Pour the marinade over the pork shoulder and toss to coat. Cover and refrigerate for at least 4 hours up to overnight.
After marinating, add the pork to the Instant Pot with one cup of the marinade. Place the lid on the pot, cover, and make sure the valve is set to sealed. Press the “manual” button and set to cook for 90 minutes.
After 90 minutes is up, let the pressure release naturally (about 20 minutes) and remove the lid from the pot.
Preheat the oven to broil.
Shred the pork with two forks, then toss with additional cooking liquid, butter, and pineapple, then spread out on a rimmed baking sheet. Place the pan in the oven and broil for 5-7 minutes, keeping a close eye on the pork until it is crisp.
In a small bowl, combine the lime juice, diced onion, jalapeno, and cilantro.
Warm the tortillas, fill with the pork and pineapple, and top with the onion relish and serve!
For slow cooker: Marinade the pork, then add to a slow cooker with 1 cup reserved marinade and cook on low for 8-10 hours or high for 4-6 hours until it shreds easily with a fork.