No, you do not have to remove the nut that holds the flywheel on. In fact, this would be pretty difficult to do with the shoes still on. Just take a small standard screwdriver and pry the clutch shoes off (prying between the shoes and the flywheel). They should pop right off with a little force. Remember though, the springs need to be pryed up off the nut too.

On the spur gear, disconnect the brake and throttle linkages at the center diff. Remove the screws (from the bottom) that hold the entire center diff assembly to the chassis and remove it. The dog bones will fall out of the assembly with no problems. Once you have the center diff removed, remove the brake assembly (including the rotor) from the yoke closest to the spur gear. The bearing support is a 2 piece design and will split. Remove the two screws holding it together, split it and remove it. Remove the bearing and now you have complete unobstructed access to the 4 screws that hold the spur gear on.