That sounds normal to me, when your truck is off and you push it backwards the spur gears should spin, as well as the clutch bell housing on the motor. If you manually flip the transmissions reverse servo/lever and should reverse it, so when you push your truck in the opposite direction it should now spin the gears in the same fashion..

If you're getting any grinding or binding of any kind, check to make sure your throttle servo isn't applying a little brakes, as sometimes if there's even a little tension on those brakes they'll bind up..

Good luck!