I would second this. You can try bringing D20 Modern into your campaign, but it won't work as well as running a general system that was designed for more diverse settings. As a "low budget" option, you can also just re-skin D&D mechanics with minor modifications: a suit of Full Plate at AC 18 becomes Heavy Body Armor at AC 18, a flamethrower can be represented with the appropriate breath weapon, a sniper rifle can be implemented as something like a crossbow with +1 die to damage, +3 damage, something like that. Spells can be re-skinned too, or you may want to create homebrew ones with unique flavors for your various settings. If you want to go the "high budget" route, pick up a general-purpose system for this kind of campaign; doing so will require some investment from the players, but I trust the recommendations of lextenebris for finding a fun one to run. He knows his stuff.