In a dry skillet over medium heat, toast the guajillo and ancho chilies for about 1-2 minutes until fragrant. Be careful not to burn them!
In a blender, combine the toasted chilies, onion, garlic, cumin, oregano, smoked paprika, salt, chicken broth, and apple cider vinegar. Blend until smooth.
Place the chicken thighs in the slow cooker and pour the chili marinade over the top. Ensure the chicken is well coated.
Set the slow cooker to low for 6-8 hours or high for 3-4 hours, until the chicken is tender and easily shreds.
Once cooked, use two forks to shred the chicken directly in the slow cooker. Mix it back into the sauce for extra flavor.
Warm the corn tortillas, fill them with the chicken mixture, and top with your choice of cilantro, diced onions, and lime juice. Enjoy!