With the front output shaft and gears in place you can install the rear shift rail. Put the front rail in Neutral so that the interlock will clear as the rear rail is installed. Then install the shift fork and set screw. The sliding gear and shift fork must go in before the intermediate gear. We use Dr Tranny's Assembly Goo to hold the needle bearings inside the intermediate gear for installation. Vaseline also works, but heavy axle grease may not fully dissolve once the T-case is up to operating temperature. The Vaseline or Tranny Goo will dissolve completely and won't impede oiling.