To run a Dies the Fire style campaign, you really don't need d20 Modern, as all the "Modern" is gone.
Magic is not (yet, apparently) a major active force... no one goes around casting spells, though something seems to happen from time to time whenever a wiccan is involved, especially the Mackenzie. There is no control over that, though, so spellcasters are not needed.
Really, everyone is a fighter of some type.
Considering the way armor works in this game, I think there are four ways to run a Dies the Fire campaign:
1) Regular D&D, but only with the barbarian, fighter, ranger, and rogue classes (and maybe monk, if you like martial arts, but without the supernatural abilities).
2) Use the Conan system (armor in DtF works more like armor in this system than in D&D) with the classes as they are, though the scholar is literally just that, a scholar with lots of general knowledge plus skills useful in a low-technological society (like Ken Larsson and a lot of the Corvallis survivors; PA low-tech knowledge means the difference between life and death).
3) Runequest, without magic. Violent, bloody, and deadly without armor, it is the most perfect fit for the combat in the game.
4) GURPS works well, too. A lot of the writing in DtF reminds me of the battles in the Horseclans series, which got the GURPS treatment many years ago...