try decoupling the motor from the drivetrain, and free roll the vehicle to see how much resistance you get. Since you are basically using the belt to connect two seperate trannies from what I recall, there will be a bit more friction I am thinking.

Other areas to look at are how tight the cases are assembled. Sometimes this can add to friction being seen.

How much slack does your belt setup have?

lastly, the drivetrain could just need to be run in more depending on how new it is.