Step 1Wash and pat the ribs dry. Remove the silver skin (the membrane on the underside of the ribs): Nudge a blunt knife or the back end of a spoon between the ribs and membrane. When enough membrane is loosened to get a good finger hold, simply pull the membrane off the rack -- it should come off fairly easily.
Step 2Lay the ribs in a glass or ceramic dish. Combine the salt, sugar, oregano and cumin and mix well, then sprinkle evenly over both sides of the ribs. Cover and refrigerate overnight.
Step 3Remove the ribs from the refrigerator, uncover them and let them come to room temperature over 2 hours.
Step 4Heat the oven to 200 degrees. In a small bowl, combine the lime juice, chipotles and oil. Wipe or rinse the ribs to remove the excess salt and sugar, and dry the meat well. Lay on a baking sheet and spoon the mixture evenly over the ribs.
Step 5Bake the ribs until they are tender (a knife inserted between the ribs will slide in with no resistance), 3 to 4 1/2 hours. Slice the ribs to separate them and serve.