For the braised lamb: Heat a medium skillet over medium-high heat. Toast the chiles skin-side up until fragrant and beginning to darken, 30 seconds to 1 minute. Transfer to a bowl and pour enough boiling water to cover and soak until softened, about 15 minutes. Strain, reserving the water. Transfer the chiles to a blender or food processor along with 1/2 cup of the reserved steeping water, the cider vinegar, cumin, oregano and garlic. Process until a smooth paste forms. Sprinkle with salt and black pepper. Place the lamb in a Dutch oven and slather the chile paste over the shanks. Add the chicken stock. Bring the liquid to a boil, and then reduce to a simmer, cover with the lid and braise until the lamb is tender, about 2 hours 30 minutes.
For the pickled peppers: Meanwhile, bring the lime juice, sugar and vinegar to a simmer in a small saucepan over medium-high heat, stirring occasionally until the sugar dissolves, about 2 minutes. Put the chiles in a separate bowl and pour the vinegar mixture over. Let cool for about 20 minutes. Discard the liquid before using.
For the guacamole: Combine the cilantro, cumin, salt, tomatillos, garlic, avocado, onions, jalapenos and lime juice in a food processor or blender and pulse until combined and still somewhat chunky.
For assembly: When cool enough to handle, remove the lamb from the braising liquid and shred. Once the lamb is removed, bring the braising liquid back to a boil and cook until it reduces to about 2 cups of liquid, 10 to 12 minutes. Return the shredded lamb back to the braising liquid in the Dutch oven. Toast the tortillas, if desired, over an open flame just until slightly charred. Fill each tortilla with some of the shredded lamb, guacamole, cilantro, radishes and pickled peppers and top with queso fresco. Repeat until all the tacos are assembled. Serve with lime wedges.