Hold the (4) wheels stationary by pushing down on the car and see if the spur gear will turn. It should NOT turn without it trying to move the car. If it does turn freely when you are holding the tires then it is either an outdrive screw loose or a diff gear(s) is/are blown. Have you taken the center diff out yet to check it ? Hold the spur gear and turn 1 outdrive and the opposite end should counter rotate. If it doesn't......blown diff gear(s).