Combine the tomatoes, chipotles, adobo sauce, onion, garlic, cumin, oregano and salt in a blender and pulse 10 seconds, until a roughly smooth sauce forms.

Place the chicken thighs in a slow cooker and pour the sauce over them, then add the chicken broth.

Cover and cook on low for 6 hours, or on high for 3 to 4 hours, until the chicken shreds easily when pressed with a fork.

Remove the chicken to a cutting board and shred it with two forks, then return it to the sauce and stir to coat every piece.

Switch to warm and let it sit uncovered for 10 minutes so the sauce thickens slightly and clings to the chicken instead of pooling.