I must assume that your drop down menu is a javascript menu, and I must assume that it uses images rather than css. If that is the case then create the images in your dropdown menu in Photoshop, Fireworks, GIMP or your favorite image editor and set the transparency down to 50% or less. Then save them with the GIF file type and code your javascript to point to the images. Then when they drop down they will appear to be translucent with the background below them showing through.
