1. Squeeze juice from orange and lime into a blender. Add garlic, salt and peppercorns. Blend until pepper is ground. Pour marinade into a wide container and put pork chops in marinade, turning to season evenly. Cover and chill chops for at least 1 hour and up to 24 hours, turning occasionally.
2. Prepare a grill or heat a grill pan. Brush grill rack or pan lightly with oil and cook pork chops (in batches, if necessary) until just cooked through, about 2 minutes per side. Transfer chops to a plate to rest for a few minutes, then thinly slice meat and mix with any juices on plate. Make tacos with tortillas, salsa, avocado, onion and cilantro.