In a small bowl, combine the garlic powder, dried oregano, chili powder, salt, and black pepper.
Rub the pork loin with the spice mixture (the whole roast should be covered).
Put the roast in the slow cooker. Add the onion, jalapeno, orange juice, lime juice, and water and stir around to combine everything.
Cook on low for 6-7 hours or high for 4-5 hours.
Shred the meat in the slow cooker (remove any large pieces of fat) and transfer to a baking sheet lined with foil (use a slotted spoon to do this, but it's okay if some of the juice makes it to the pan).
Broil on high in the oven for 4-5 minutes. Keep an eye on the meat, though; it can burn easily under the hot broiler!
Remove from the oven and spoon a little bit of the juice from the slow cooker over the meat.
Serve with warmed corn tortillas, red onion, cilantro, and a squeeze of lime.