A friend of mine owns a 400 Xplorer and he wants to have more top end but does not want to lose too much bottom. Would it be possible to only change the rear drive sprockets and gain speed but still have enough low end grunt in 4WD to get up big hills?

 

Sorry but you have to keep the gear ratio thhe same for the 4x4 to work right.What he needs is to pipe,clutch and port the motor along with a good set of reeds.