Heat oven to 300° F.
Place 2 large sheets of aluminum foil on a work surface, one atop the other. Place the pork in the center of the foil and season on all sides with the salt, pepper, and cumin. Spoon the jam over the pork, turning to coat. Scatter the jalapeños and onion around the pork. Fold the double layer of foil around the pork and crimp the edges to seal tightly. Place in a baking dish. Roast until fork-tender, about 4 hours; let cool. Unwrap the pork, reserving the juices and discarding the foil. Use a fork to shred the pork. Transfer to a bowl. Pour the cooking juices, onion, and jalapeños over the pork and toss. Serve with the tortillas, cilantro, and lime wedges.
Tip: The best place to warm the tortillas is in the oven: it's quicker than a skillet and less drying than a microwave. As soon as you remove the pork, turn off the heat, wrap the tortillas in foil, and warm them for 10 to 20 minutes.