Check to see it the rear center cvd pin is hitting in the brake rotor. The center pins are 1mm shorter than outdrive axle ones and if wrong one happened to get pressed in at factory, it would bind. Also check to make sure that the brakes are not hanging up or that under full throttle, if the front or rear brake linkage is not set correctly causing the brakes to come on slightly.. I did that on first one I ever built..
Also make sure that a front or rear diff is not shimmed to tight..