Preheat oven to 350°F with a rack in the bottom third. On a plate, mix the flour with 1 teaspoon salt and 1 teaspoon pepper, and dredge and coat the pork shanks in the mixture, shaking off excess flour.
In a small bowl, combine the chipotles in adobo, chipotles in adobo sauce, vinegar, tomato paste, oregano, and the remaining ½ teaspoon salt and pepper.
Heat the oil in a large ovenproof casserole over high heat. Once hot, brown the shanks a couple minutes per side, about 8 to 10 minutes altogether. Transfer to a bowl or platter.
Reduce heat to medium, add the onion, celery and carrots and cook for 4 to 5 minutes, stirring occasionally, until softened and beginning to brown. Add the garlic and cook for another minute until fragrant. Pour in the beer and bring to a simmer, scraping up the bits and pieces from the bottom of the casserole with a wooden spoon. Simmer for about 3 minutes until the alcohol cooks off.
Pour in the broth, bring to a simmer, and add the chipotle mixture and stir well. Add the browned pork shanks, spooning some of the cooking sauce and vegetables over them. Cover and place in the oven for 2½ to 3 hours, until the meat is completely tender and falling apart.
Remove from the oven and serve with warm corn tortillas and guacamole.