For the pork: Sprinkle the pork with the salt and pepper. Heat the oil over medium-high heat in a 5-quart saucepan. Cook the pork until brown, about 10 minutes. Add 3 cups water, the chicken broth, orange juice, cumin, chiles, garlic and onion. Bring the mixture to a boil. Reduce the heat to a simmer and cook until the pork is very tender, for 2 hours. Shred the pork into small pieces, using 2 forks. Season with salt and pepper.
For the salsa: Cut the ends off the oranges and grapefruits, place cut-side down on the counter, and cut away the peel. Using a paring knife, cut along the membrane on both sides of each segment. Coarsely chop the fruit segments. Squeeze the juice from the membranes and reserve. Whisk together 2 tablespoons of the reserved citrus juice, oil, honey, vinegar, salt and pepper in a small bowl until smooth. Add the fruit segments and toss until coated.
To serve: Warm the tortillas by holding them over a burner for 30 seconds on each side or place in a dry skillet and heat over medium heat until warmed through. Place 1/4 cup of shredded cabbage in the center of each tortilla and place the pork on top. Spoon the salsa over the meat and serve.
Any leftover meat can be used for sandwiches, quesadillas or tossed with cooked pasta.