Place chicken breasts in a crockpot. Sprinkle with taco seasoning and cover with salsa. Cover the crockpot with lid, and cook on high for 4-5 hours, or low for 6-8.
Remove chicken from the crockpot, and use a fork to shred. Return to crockpot and stir into salsa.
Serve chicken on whole wheat soft-taco shells, topped with guacamole and Greek yogurt. Garnish with chopped tomatoes and cilantro.