Drain the pineapples and place juice in a small saucepan with about 1/2 cup of water. Bring to a boil; remove from heat. Add chiles and let them soak for about 30 minutes.
When chiles are softened, remove stems. Place chiles and soaking liquid in blender. Add 1/4 cup of the pineapple chunks, garlic, oregano, vinegar and achiote paste. Puree until smooth. Cover and place remaining pineapple chunks in refrigerator for later use.
Remove pork from the freezer and slice in 1/8th to 1/4 inch slices, depending on your preference. Place pork slices in a plastic resealable bag. Pour the marinade over meat and refrigerate for at least one hour or up to overnight.
Prepare grill to medium heat. Meanwhile, boil used marinade for about 10 minutes. Add the cornstarch and water mixture. Simmer on low until thickened. Grill meat about 2 to 3 minutes per side. Watch them close because they are thin and will cook quickly. When done, plate meat and pour glaze over the top. Cut up the remaining pineapple chunks and serve over the meat, if desired. Spoon meat into tortillas and top with onion, cilantro and sour cream.