In a 6-quart saucepan, combine the tomatoes, molasses, onion, sherry, Worcestershire sauce, chipotles, garlic, allspice, and liquid smoke. Add salt and pepper to taste. Bring to a simmer over medium-high heat.
Reduce the heat to low and cook, uncovered, stirring occasionally, for 30 minutes. Use the immersion blender to mix the ingredients into a smooth sauce. (Or transfer to the stand blender and puree for about 60 seconds.) Simmer, stirring frequently, for an additional 45 minutes or until the sauce is reduced to about 2 quarts.
Rub the meat with salt and pepper and put it in the slow cooker. Add about 6 cups of the barbecue sauce, then cover and cook on high for 7 to 8 hours or on low for 10 to 11 hours. The meat should be fork-tender and falling apart.
Remove the meat and place in a large bowl to cool.
After the cooking liquid has sat for a few minutes, strain the fat.
When the meat is cool enough, shred it with your fingers, pulling off the remaining fat and gristle.
Toss 2 to 3 cups of the cooking sauce with the meat, and add more as needed to keep it moist. Serve on buns and pour the reserved sauce on top.