I play it on a 5er so for the dropped notes I just move down a string.
Are you transcribing it or just wanting to know what to do with the low D? You could just bump the D up an octave (A string for example).
If you're reading if off of tab, subtract 2 from the numbers on the lowest string. So if it says 4, you play it on 2 on the E.
Like in this tab:
http://www.bassmasta.net/a/adams,_bryan/101574.html
Wherever is says 0 for open D, just play the D somewhere else (easiest on the A string). Won't quite have the growl of being in drop D, but may blend better.
