In a medium bowl combine: 2 cups brown sugar, 2 cups orange juice, 3 tablespoons orange zest and 8 ounces of orange marmalade, whisk to combine.
If the ham is not pre-sliced, slice it, one inch from the bottom/flat side, so the slices are still holding together. Place ham flat side down in the slow cooker. Pour the brown sugar orange mixture over it.
Spread the remaining orange marmalade and Boar’s Head Brown Sugar & Spice Glaze in between the slices.
Top the ham with the remaining ½ cup of brown sugar, rub it well into the ham. Top with orange slices.
Put the lid on slow cooker and cook on low for 4 hours, or high for 2 hours. Since the ham is pre-cooked, you're are only heating it up and melding the flavors into it. Cooking too long will just dry it out.
During the cooking process, lift the lid a few times and using a ladle pour the juices over the meat in order to keep it moist.
Once ready, slice ham and serve.
Store it in an airtight container filled with the brown sugar orange glaze mixture from the crockpot.