To make the marinade: In a large glass bowl, whisk together the olive oil, lemon juice, orange juice, chili flakes, chili powder, garlic, salt, and black pepper. Add in the pork and toss to coat. Cover the bowl and refrigerate for at least 30 minutes (although I prefer to marinate for 3 hours or overnight).
Soak the wooden skewers in a shallow pan of water for at least 1 hour.
Remove the pork cubes from the marinade. Thread 3 or 4 pieces of marinated pork on the skewers, alternating the pork with small wedges of lemons and oranges. Place the skewers on an area of the grill with no direct flame. Continue to brush marinade during grilling, until the meat is browned, about 8 to 10 minutes, turning occasionally or to desired doneness. Transfer the pork from the skewers and place in pita pockets. Top the pork with shredded lettuce and avocado tzatziki.
In a large bowl, whisk together the lemon juice, olive oil, garlic and salt. Whisk in the yogurt. Add the grated cucumber, diced avocado and mix well. Cover the mixture and chill for at least 1 hour.
Cook's Note: To get as much cucumber flavor as possible, grate the cucumber right over the yogurt.