Using a small, sharp knife, make random 1 1/2-inch slits in the pork roast and stuff each with a prune. Heat the olive oil in a Dutch oven or ovenproof stockpot over medium-high heat and brown the pork on all sides. Remove the pork from the Dutch oven; set aside.
Meanwhile, in a food processor or blender with the motor running, process the garlic cloves until minced. Add the onion and pulse until minced.
Heat oven to 325° F. Add the minced garlic and onion to the Dutch oven and sauté until tender but not browned. Add the apple juice, cider vinegar, chili powder, salt, oregano, and cinnamon. Return the park to the Dutch oven. Over high heat, bring to a boil. Cover and place in oven. Cook 1 1/2 hours, basting occasionally. Remove the cover and cook 30 minutes longer.
Remove the pork from the sauce. Set the sauce aside for 10 minutes. When the fat comes to the surface, skim it off and discard. Boil the sauce over high heat, stirring occasionally, until thickened and reduced to about 1 1/2 cups. Slice the pork roast and serve with the sauce.
KEEP IT: Slice any leftover pork into 1/2-inch-thick pieces and store in airtight containers or heavy-duty zipper bags with some of the sauce. Refrigerate for up to 3 days or freeze up to 3 months. To reheat, microwave the pork in the sauce so that the meat remains moist.