In a small saucepan, bring vinegar and sugar to a boil for 1 minute. Place onions in a bowl; pour vinegar mixture over onions; chill until ready to serve.
In a blender, puree avocados with sour cream, half of the lime juice and salt to taste; set aside. In a small bowl, combine jicama, cabbage, half of the oil (1 tablespoon), the remaining lime juice (1/2 lime) and a pinch of salt; toss.
Season shrimp with blackening spice. Heat 1 tablespoon oil in a large pan to smoking hot, add shrimp; saute until cooked through, about 2-3 minutes.
To serve, warm tortillas and fill with jicama salad, shrimp, avocado cream, red onions (from vinegar mixture) and 1 sprig of cilantro.