Preheat the oven to 325 degrees . In a large dutch oven, heat 2 tablespoons vegetable oil over medium-high heat. Season a 5-pound pork shoulder roast with salt and pepper and cook until browned on all sides, about 8 minutes. Add 4 crushed garlic cloves and cook for 30 seconds. Stir in 1 cup each grapefruit juice and water and 1 tablespoon grated grapefruit peel; cover and transfer to the oven. Cook until fork-tender, about 2 1/2 hours, then shred. In a medium bowl, combine the segments of 2 grapefruits and their juice, 2 cups chopped red onion, 1 chopped avocado, 3/4 cup chopped cilantro and 1 seeded and finely chopped habanero chile; season with salt. Divide the pork and salsa among 8 hamburger buns.