Heat a small, heavy skillet over medium heat. Add the cumin seeds and peppercorns to the pan; stir constantly until fragrant and beginning to brown, about 2 minutes. Cool.
Using a mortar and pestle, crush toasted spices with garlic, salt, and oregano to make a paste. You can also do this in the small bowl of a food processor. Transfer to a small bowl, and stir in orange juice, lime juice, lemon juice, sherry, and olive oil.
Place the pork in a large resealable plastic bag. Pour citrus marinade over meat, and seal. Refrigerate for 12 to 24 hours, turning the bag over occasionally.
Preheat the oven to 325 degrees F (165 degrees C).
Transfer pork and marinade to a roasting pan, and place in the oven. Roast for about 2 1/2 hours, basting with pan juices occasionally, or until an instant read thermometer inserted in the center reads 145 degrees F (63 degrees C). Add small amounts of water to the pan if it dries out. Transfer the pork to a carving board, cover loosely with foil, and let rest for 15 minutes. Carve, and serve.