Preheat the oven to 325° and position a rack in the bottom third of the oven. Set the smoked ham in a large roasting pan. Score a 1/4-inch-deep crosshatch pattern into the fat at 2-inch intervals. Pour 2 cups of the Dr Pepper and the 2 cups of water into the pan and roast the ham for about 2 1/2 hours, until an instant-read thermometer inserted in the thickest part of the ham registers 120°. Increase the oven temperature to 425°.
Meanwhile, in a medium saucepan, simmer the prunes in the remaining 1 cup of Dr Pepper until they are plump and the liquid is slightly reduced, about 10 minutes. Using a slotted spoon, transfer the prunes to a small bowl and cover with plastic wrap. Whisk the mustard, brown sugar and vinegar into the liquid in the saucepan and boil until very thick and syrupy, about 5 minutes.
Drizzle the syrupy glaze over the ham and roast until glossy, 20 minutes. Carefully transfer the ham to a cutting board.
Pour the pan juices into the saucepan and spoon off the fat. Boil the sauce until reduced to 2 cups, 10 minutes. Whisk in the cornstarch slurry and the prunes and bring to a boil. Simmer the sauce until thickened, 2 minutes. Slice the ham and serve with the Dr Pepper sauce.
The sweet, fruity Dr Pepper glaze on this juicy ham pairs best with an equally easygoing, fruity wine—Zinfandel, for instance. California’s Lodi region is a source for good Zinfandel.