In large glass jar, stir together sugar, salt, and 1/2 cup boiled water until sugar and salt are dissolved. Add cider vinegar and let cool to room temperature. Add berries and red onion. If needed, add additional cider vinegar so everything is covered. Seal lid and let stand for at least 1 hour or up to 3 days in refrigerator.
Build medium-hot fire in charcoal grill or heat gas grill to medium-high. Brush pork with oil. Grill pork, turning a couple of times, until internal temperature of 145 F is reached, about 12 minutes. Set aside to rest for 5 minutes and then thinly slice.
Remove berries and onion from pickling brine and place in bowl with tomatoes, bell pepper, and cilantro.
In blender, blend together sour cream, avocado, lime juice, and cayenne until smooth.
To assemble tacos, place pork on tortillas and top with berry salsa and avocado crema.
Per serving: 406 calories; 31 g protein; 12 g total fat (3 g sat. fat); 47 g total carbohydrates (11 g sugars, 9 g fiber); 521 mg sodium