See scan of the transmission from the Fleetline Shop Mauual. The shifter fork that moves the left hand coupler will be either to the left or right depending where the shift lever was last placed. Shift fork that moves #16 coupler to the left is R1 or R2. When the fork moves the coupler to the right it is either 1st or 3rd gear. Shift fork that moves cuupler #21 is moved to the left it is either 2nd or 4th gears. When the fork moves the coupler to the right it is either 5th or 6th gears.
For "N" both the center coupler and the rear coupler must be in the centered position. Should you find both the center and rear coupler centered, check the ring gear for backlash, you should get a slight movement between the ring gear and pinion shaft. If the couplers are centered and you have backlash on the ring gear and it is still locked up you have a gear bushing siezed to the pinion shaft.
These transmissions are bullet proof, it must have been run out of oil.