Melt butter in a large heavy saucepan over low heat; whisk in flour. Cook, whisking constantly, 1 minute. Gradually whisk in milk; cook over medium heat, whisking constantly, until thickened and bubbly. Stir in onion and next 4 ingredients. Remove from heat, and stir in 1 egg and shrimp. Cover and chill 1 hour.
Shape shrimp mixture into 16 balls. Roll in saltine crumbs, dip in 2 beaten eggs, and roll again in saltine crumbs.
Pour oil to a depth of 3 inches into a Dutch oven, and heat to 375°. Fry croquettes 3 to 4 minutes or until golden. Drain on paper towels.