Place guajillo chilies in a small saucepan. Cover with water and bring to a boil. Remove from heat and let steep for 30 min. Remove stems and seeds and squeeze out excess liquid.
Puree guajillo chilies and all other marinade ingredients in a blender or food processor until smooth. Place pork in a non-reactive, ovenproof pan, such as glass or stainless steel, and pour half of marinade over it. Mix well and marinate pork in fridge for 4-6 hrs or overnight. Reserve remaining marinade.
Add 3 c pineapple to the reserved marinade in the blender and puree until smooth. Transfer to a saucepan, bring to a boil, then reduce heat to low. Simmer for 10 min, until the sauce has thickened.
Preheat oven to 325°. Add chicken broth and ½ c pineapple sauce to the pork. Cover the pan with parchment paper and foil, and then place in the oven for 2-2 ½ hrs until pork is very tender. Remove pork from the cooking liquid and set aside.
For pineapple relish, mix 1 c grilled and diced pineapple, onion and cilantro in a small bowl. Add a pinch of salt and set aside until ready to use.
Heat oil in a large, heavy bottomed nonstick skillet over high heat, then briefly sauté the pork in batches, allowing the pieces to caramelize but taking care not to let the sauce burn.
Divide the pork between 12 warmed tortillas, garnish with cotija and pineapple relish. Serve warm with a wedge of lime.