Replace the D string in your set of GHS Guitar Boomers Low-Tuned Strings with the DY33. This string is meant to sit in the 4-D position of your guitar, but practically speaking, it's not a D string per se: GHS Guitar Boomers are meant to give you a hefty, powerful tone in multiple low tunings.
GHS accomplishes this by employing a gauge and core winding much heavier than the common electric guitar string. So, whether you tune a whole step or even a perfect fourth down, you can rely on this string to provide a clear, distinct, and present baritone tone.