Cook:10 hrs to 12 hrs (low) or 5 to 6 hours (high)
For marinade, in a small bowl combine lime juice, water, grapefruit juice, garlic, oregano, salt, cumin, pepper, and bay leaves. Trim fat from meat. If necessary, cut roast to fit into slow cooker. With a large fork, pierce meat in several places. Place in a large plastic bag set in a deep bowl or a baking dish. Pour marinade over meat. Close bag. Chill in the refrigerator for 6 to 24 hours, turning occasionally.
In a 3-1/2- to 5-quart slow cooker place onion. Top with meat and marinade mixture.
Cover; cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours.
Transfer meat to a cutting board; cool slightly. Skim fat from juices; keep warm. Remove bay leaves; discard. Use 2 forks to gently separate the meat into shreds. Transfer shredded meat to a serving platter. With a slotted spoon, remove onions from juices. Transfer onions to same serving platter. Serve meat and onions in tortillas with small bowls of the hot juices and Pico de Gallo. If desired, pass lettuce and guacamole. Makes 8 to 10 servings.
Pico De Gallo
In a medium bowl combine 2 peeled and finely chopped medium tomatoes, 2 tablespoons finely chopped red onion, 2 tablespoons snipped fresh cilantro, 1 teaspoon lime juice, 1/8 teaspoon salt, and dash sugar. Mix well. Cover; chill in the refrigerator for at least 2 hours or overnight. Makes about 1-1/4 cups.