[Overdriven]

50k in a front or rear diff should be putting the power down pretty evenly as that is thick even for a MT. I think it's just spinning the tires and drifting due to the gyroscopic effect of the motor and driveshafts.

Either that or something is off in the steerig and/or alignment dept. Emaxx servo savers are notoriously sloppy, use the black spring in the servo saver and double check the alignment. Also with the truck off the ground give it a little stab of throttle, see if the wheels steer themselves at all when doing so. You could give the left rear a little more toe than the right rear to help it track a little straighter if all else fails.