Good luck working that out. You will need to do some heavy modding I can tell you that much.
In an Inferno, you can toss a Landmax 2spd, or perhaps Mad-Force 3spd - sacrificing the center diff - with a few mods. You could try those options, but its nothing definate.
Ofna also make 2spd units for their older 1/8 buggies, which could be options too.
Just curious - what version of the X4 are you running (reg. X4, X4RR, X4XR) and what engine are you using? A new engine, new clutch, or higher gearing might be a better way to get more speed than trying to get 2spd's in there. Having multiple gears will limit you offroad like nothing else.