Place broth, orange juice concentrate, chipotle, adobo, and garlic in a blender or food processor. Blend until smooth.
Place chicken in a 4-qt. or larger slow cooker and season with salt. Pour the sauce over the chicken. Cover and cook on Low until the chicken is tender and can easily be pulled apart with a fork, 4 to 5 hours.
Transfer the chicken to a cutting board. Let cool slightly.
Carefully transfer the cooking liquid to a medium saucepan. Bring to a boil over medium-high heat. Cook until reduced to ¾ cup, about 15 minutes. Return the sauce to the slow cooker.
Meanwhile, prepare salsa: Zest orange, then peel; chop the segments. Mix onion, cilantro, lime juice, the orange zest, and the chopped orange in a medium bowl. Add avocado and toss gently until just combined.
Using 2 forks, shred the chicken. Return it to the slow cooker and stir to combine with the sauce. To serve, divide the chicken among tortillas and top with cabbage and the salsa.
