Prepare the ham: Combine 3 gallons hot water, the salt, brown sugar, juniper berries, peppercorns, ginger and bay leaves in a large nonreactive container or pot and stir until the salt and sugar dissolve. Let the brine cool. Meanwhile, stand the ham, cut-side down, on a cutting board and score the skin with a sharp knife in a crosshatch pattern. Place the ham in the brine, cover and refrigerate at least 24 hours or up to 2 days.
Preheat the oven to 450 degrees F. Remove the ham from the brine and dry completely. Strain the brine, reserving the spices. Press some of the juniper berries and peppercorns onto the flat cut side of the ham. Scatter the bay leaves in a large roasting pan and place a rack in the pan. Stand the ham, cut-side down, on the rack and roast 1 hour. Reduce the oven temperature to 325 and roast 2 more hours.
Make the glaze: Whisk the peach preserves, brown sugar, mustard, vinegar, ginger, and salt and pepper to taste in a bowl. Brush the ham with half of the glaze; continue roasting until deep golden, about 1 more hour. Brush the ham with the remaining glaze, turn it on its side and roast until a thermometer inserted into the center registers 155 and the skin is crisp, 30 to 45 more minutes. Transfer to a cutting board, tent with foil and let rest 20 minutes before carving.
Skim the fat from the roasting pan juices and discard the bay leaves. Stir to make a sauce. Carve the ham and serve with the sauce.