Add everything but the chicken to a food processor or blender. If you're sensitive to spice, use half the can of chipotle peppers. Blend until smooth and add to the crockpot with the chicken. 

Flip the chicken a few times to fully coat both sides.

Cover and cook on low for around 2 to 2.5 hours or until the chicken reaches an internal temp of 165F. Be careful not to overcook or the chicken will be difficult to shred. (You can cook on high, but the longer cook time will produce more flavorful chicken.) 

After the chicken is cooked, shred it with two forks or use a stand mixer with a flat beater attachment. (You can also chop the cooked chicken with a knife.)

Set the crockpot to its keep warm function and add the chicken back to the crockpot. Let the chicken rest in the juices for an additional hour.