While not normally drained during routine oil changes, the shaft drive middle gear case is equipped with a drain plug for those times you plan to tear down the transmission, or the oil has become contaminated. It's a PITA to locate and remove. It's at the rear of the engine, left hand side, inboard of the frame tube. It's very difficult to remove with the stock exhaust system in place and even more difficult to re-install. A 1/4" drive 10mm socket, u-joint, and long extension are required. With the exhaust system removed, the drain is far easier to access.
