Remove the stem and seeds from the chile ancho. Combine the chile ancho and the chile de arbol in a sauce pan and cover with water. Bring to a boil, reduce heat and continue cooking for 10 minutes.
Drain the chiles and transfer to the blender. Add all of the remaining ingredients for the marinade. Blend on high until smooth and taste for salt. Pour all of the marinade over clean jumbo shrimp and stir to coat evenly. Cover and marinate for no more than 1 hour.
After an hour, remove the shrimp from the marinade and shake off excess marinade. Discard marinade. Transfer shrimp to a plate and let it come to room temperature. Heat your outdoor or stove top grill to medium/high heat. I highly suggest you use a pan on top of the grates of an outdoor grill or easy-release foil paper if using a stove-top grill pan.
While the shrimp comes to room temperature, grill the pineapple, red onion and fresh chiles. Drizzle with a little olive oil and grill for a few minutes per side or until desired consistency. Remove from heat and set aside.
If using a stove-top grill pan, add a new piece of easy-release foil paper and drizzle with oil. Cook the shrimp for about 3 minutes per side. Transfer to plate and tent loosely with foil paper.
Roughly chop the pineapple, red onions and chile peppers and transfer to serving dish. Serve the shrimp on tostadas with your favorite salsa, avocado, cilantro, lime, shredded cabbage and grilled pineapple/onion mixture.
This marinade for al pastor can be used for smaller cuts of pork, such as ribs or pork chops as well. It freezes well and will improve in flavor the longer it is stored.