Place pork tenderloin in slow cooker. Add onion, enchilada sauce and pineapple. Cover and cook 6-7 hours on low in slow cooker.
After 6-7 hours, remove pork and shred using 2 forks. Place shredded pork back into slow cooker, cover and return setting to low for 30 minutes.
After 30 minutes, stir in lime juice and cilantro.
To assemble, place shredded mixture into tortillas or taco shells with desired toppings.