Place tomatoes in slow cooker and crush with hands or a wooden spoon. Add onion, garlic, chipotle peppers, thyme and basil. Stir with a spoon until well blended. Place chicken in slow cooker, breast side down. Season with salt and pepper. Cook on high for 3-3.5 hours or low for 6-7 hours.
To make the crema, mix together sour cream, milk, salt, and lime zest and juice in a small bowl. Cover with plastic wrap and keep chilled until ready to eat.
When chicken is finished cooking, remove chicken from slow cooker and allow to cool slightly. Pull meat into bite size pieces and discard skin and bones.
Mix all or some of the sauce from the slow cooker in with the chicken (I used some and reserved the rest for another use).
Top tortillas with pulled chicken, lettuce, tomatoes, crema, and any other desired garnishes.