In medium bowl, beat softened butter and 2/3 cup sugar with electric mixer on low speed 30 seconds; increase speed to high, and beat 2 to 3 minutes, scraping bowl occasionally, until light and fluffy. Reduce speed to low; beat in eggs, one at a time, just until smooth. Beat in vanilla and grapefruit zest. On low speed, beat in one-third of the flour mixture, then half of the milk, beating well after each addition, just until smooth. Repeat with flour mixture and milk, ending with flour mixture. Scrape side of bowl occasionally. Beat in sour cream until blended. Divide batter among loaf pans.