Place the roast in a large pan with water to cover and simmer until tender and the meat begins to fall apart, about 3 to 4 hours. Remove the roast from the pan, remove the fat and bone, and shred the meat by hand or with a fork.
Return the meat to the pan, add the remaining ingredients, stir well, and simmer until all the liquid has been absorbed by the meat. Place the meat in a large flour tortillas with the tomato, lettuce, cheese, and sour cream, if used.
