Using tongs, briefly toast the guajillo chiles over an open flame or in a cast-iron skillet until fragrant, 5 seconds per side. Transfer the guajillos to a blender. Add 2 cups of hot water and let stand for 15 minutes.
Add the garlic, chipotles, black pepper, cinnamon and oregano to the blender and puree. Strain the sauce into a large enameled cast-iron casserole, pressing on the solids. Add the pork and 6 cups of hot water to the casserole; bring to a boil. Cover partially and simmer over low heat until the pork is tender, 2 hours.
Using a slotted spoon, transfer the pork to a bowl and let cool slightly. Boil the sauce until it is reduced to 4 cups, about 20 minutes.
Shred the pork with 2 forks and return it to the sauce. Simmer uncovered until the sauce is reduced and just coats the pork, about 20 minutes. Season the pork with salt and let cool slightly.