Soak the ancho chiles in boiling water until softened, about 15 minutes. Drain and discard the liquid. Transfer the softened chiles to a blender. Add the onion, garlic, brown sugar, apple cider vinegar, beef broth, 1 teaspoon salt and 1 teaspoon pepper. Puree until smooth.
Pat ribs dry and sprinkle with salt and pepper. Heat the oil in a 12-inch heavy skillet over medium-high heat until hot but not smoking. Brown the ribs in 3 batches, turning occasionally, about 7 minutes per batch. Transfer the browned ribs to a slow cooker.
Pour the sauce over the ribs (liquid should reach about halfway up sides of meat),
cover and cook for 7 hours on high until very tender. Allow the ribs to cool slightly in the sauce.
Remove the ribs from the sauce. Discard the bones and shred the meat into a large bowl. Skim the fat from the surface and discard. Mix the sauce into the meat and season with salt and pepper, to taste. Spoon the meat and sauce into warm corn tortillas and serve with Mexican crema and chopped fresh cilantro.
The ribs improve in flavor if cooked 2 days ahead. Cool completely, uncovered, then chill, cover and refrigerate. Remove any solidified fat before reheating. These ribs can also be cooked in a 350 degree F oven for 3 1/2 hours.