In a food processor (or by hand) chop the red onions, jalapenos and garlic into small pieces. Place the chopped onion/pepper mixture into a large bowl or tupperware. Add the reamining spices and orange/lemon juice and whisk together. Remove about 1/2 cup of the marinade and reserve for later. Place the boneless pork into the marinade. Make sure all of the pork is covered and then cover the bowl with saran wrap or a lid. Marinate the pork overnight or up to 24 hours. If you don't have that long, you can place it directly into the slow cooker but it won't be quite so flavorful. When you are ready to cook, place the pork and marinade into a large slow cooker. Turn the slow cooker to high heat and cook for 4-6 hours or on low heat for 8-10 hours. Check the pork after the minimum hour of cooking time and if the pork does not fall apart, cook for another hour or two. You want the pork to shred easily and be completely tender. Once the pork is cooked, remove from the juices and shred into a bowl. Add the fresh, reserved marinade over the pork. Add salt and pepper to taste. If you want more sauce, add some extra sauce from the slow cooker back into the pork. Serve the warm pork in corn tortillas with cheese, salsa, cilantro, lettuce, etc.