Whisk together first 6 ingredients in a shallow dish or large heavy-duty zip-top plastic bag; add chicken. Cover or seal, and chill 1 to 2 hours, turning occasionally.
Remove chicken from marinade, discarding marinade.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 20 to 25 minutes or until done. Cut 8 chicken breast halves into thin strips. Cover and chill remaining 4 chicken breast halves, reserving for Quick Chicken Tostadas.
Top tortillas evenly with chicken, avocado, and next 4 ingredients; roll up, and serve with sour cream and salsa.