Pat the lamb leg dry with paper towels. Trim away any excess or unwanted fat. Set the lamb aside.
In a medium bowl, combine the spices, molasses, vinegar, and olive oil. Whisk until well blended.
Pour the marinade evenly over the lamb leg, ensuring all sides are coated. Cover and marinate in the refrigerator for at least 6 hours, or preferably overnight.
Remove the lamb from the marinade (reserve any excess marinade). In a large skillet or Dutch oven over medium-high heat, sear all sides of the lamb until a rich, brown crust forms.
Transfer the seared lamb to a slow cooker. Pour any reserved marinade over the lamb. Cover and cook on high heat for 4 hours, or on low heat for 6-8 hours, until the lamb is tender and easily shreds with a fork. (No additional liquid is needed.)
Using two forks, shred the cooked lamb into bite-sized pieces directly in the slow cooker.