In a small bowl stir together the spice rub (onion powder, garlic powder, cumin, smoked paprika, salt, pepper, chili powder, cayenne pepper and brown sugar).
Sprinkle the pork ribs on all sides with the spice mixture.
Heat the oil in a large Dutch oven until hot but not smoking. Add the pork, sear on all sides until browned. Add the onions, tomatoes stout and smashed garlic. Bring to a boil. Reduce heat to maintain a simmer, add lid at a vent.
Simmer, turning ribs occasionally until fork tender, 3 to 4 hours.
While the pork is cooking make the pickled jicama. In a pot over medium high heat add the vinegar, salt and sugar, cook just until the sugar and salt has dissolved, remove from heat. Stir in the beer and ice water, allow mixture to cool. Add the jicama to a bowl, pour pickling liquid over, cover and chill for at least 2 hours, drain.
Once pork is fork tender, turn off heat. Using two forks, shred meat and remove bones. Allow shredded pork to sit in the simmering liquid for ten minutes. Remove with a slotted spoon to drain.
Fill the tacos with pork, top with pickled jicama, corn, cotija, avocado and cilantro.