In skillet over medium heat, add pork. If pork is in its own juices, cook over medium heat until juices are almost evaporated. Add fajita seasoning and pineapple juice and cook for 2-3 minutes. Add lime juice and cook for another 1-2 minutes. Remove from heat and add pineapple chunks.
Meanwhile, warm tostada shells according to box directions. If warming in oven or toaster oven, watch carefully to ensure they don't burn
Place warm tostada shells on plate. and top with pork and pineapple mixture. Top with tomatoes, onion, avocado, queso fresco sour cream, and cilantro. Add another squeeze of lime juice on top