In large bowl, place chicken breasts; add margarita mix and 1 tablespoon lime peel. Cover; refrigerate 30 minutes to marinate chicken.
Heat gas or charcoal grill. Drain chicken; place on grill over medium heat. Cover grill; cook 10 to 15 minutes, turning and brushing with marinade 2 to 3 times, until juice of chicken is clear when center of thickest part is cut (at least 165°F). Discard any remaining marinade.
In small bowl, mix yogurt and lime peel. Serve chicken topped with yogurt mixture and lime slices.