To prepare pulled pork, combine first 9 ingredients in a large heavy-duty zip-top plastic bag. Add pork to bag. Seal bag, turning to coat. Marinate in refrigerator overnight, turning occasionally.
Preheat oven to 325°.
Remove pork from marinade, reserving marinade. Place pork in a large Dutch oven; pour marinade over pork. Cover and bake at 325° for 4 hours or until pork is fork-tender.
To prepare pickled onions, while pork cooks, place red onion and next 4 ingredients (through jalapeño pepper) in a medium bowl, tossing to coat. Cover and marinate in refrigerator at least 2 hours.
Remove pork from pan, and place on a cutting board. Reserve 1/2 cup cooking liquid; discard remaining cooking liquid. Remove pork from bones; place in a large bowl. Shred pork with 2 forks. Stir in reserved cooking liquid.
Place 1/4 cup pork mixture on each tortilla. Top with about 1 tablespoon Guacamole and about 3 tablespoons pickled onions.
Tip: This pork is best when made ahead and refrigerated for a few days to blend the flavors. Then you can scrape off any excess fat before reheating it on the cooktop over low heat.
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