In a large bowl, whisk together flour, sugar, baking soda, baking powder and salt. In a separate bowl, whisk together yogurt, eggs, vanilla, rosemary, oil, grapefruit juice and milk. Make a well in the dry ingredients then pour in the yogurt mixture. Use a light hand and a rubber spatula to gently combine the two, and don't over-mix. Divide batter among paper cups and bake for 18-22 minutes, until tops spring back when gently pressed in the middle. Transfer muffins to wire racks to cool completely.