In a food processor combine garlic, allspice, black peppercorns, achiote paste, chili powder, oregano, bay leaves, orange juice, vinegar, and salt.
Process until all ingredients are pureed and garlic, allspice, and peppercorns have been ground fully.
Place pork in a large ziplock bag or marinating dish and marinate at least 3 hours and up to overnight.
Preheat the oven to 350 degrees.
Line a baking dish with 2 pieces of parchment, criss-crossed, with enough overhang to completely wrap the pork.
Pour pork and marinating juices into the center of the parchment, fat side up.
Gently wrap the pork so that all the liquid stays inside the parchment. (This mimics the traditional banana leaves that are used to wrap the pork during cooking).
Cover the entire dish with tin foil; pierce with a knife a few times to create steam vents.
Bake for 3 hours.
Remove from oven, uncover, and remove pork from baking dish.
Using 2 forks, shred the meat until it is the texture of very fine tendrils.
Pour some of the cooking liquid over the shredded pork to it retrains it's moisture.
Serve meat on corn tortillas and topped with Yukatan onion pickle.