For the pureed watermelon rind, you want to cut off the green skin and about 1/2 inch of the hard white part. Keep the part that is lighter red to white (this is the tender part of the rind).
Put the rind inside the food processor and purée for only about 10 seconds. Strain the liquid from the pulp and use 1 cup.
Combine the 1 cup watermelon pulp with the remaining ingredients in medium saucepan over high heat. Bring to a boil, and then reduce the heat and simmer, covered, for about 1 hour or until your desired thickness.
Cut the racks of ribs into plate-size portions, about 6 to 7 ribs each. Brush each rib with the sauce and wrap each rack individually in aluminum foil. Be sure to save some sauce for later.
Let the ribs marinate in the refrigerator for a couple of hours (overnight is best).
Preheat the oven to 300°F to cook the ribs. Set them with the seam of the foil facing up in the oven. Bake for 2 to 2 1/2 hours or until the rib meat has pulled back from the cut end of the bones by about 1/2 inch. They should be very tender.
Remove the ribs form the foil and brush with additional sauce. Grill on a hot barbecue for 2 to 4 minutes per side or until you see several spots of charred sauce (be sure to watch them so they do not burn).
Serve with reserved sauce on the side.