Hi Jacob, thanks for asking - there is no such thing as too many questions
You are referring to part number P-1010 the 'sliding clutch gear'
It slides in through part P-1009, the 'gear clutch plate'. This is toothed, and the sliding clutch gear must be rotated for the teeth to engage in these teeth or it won't slide in. Have a look at this diagram:
app.php/gallery/image/2503
The sliding clutch gear also has to mesh with the teeth on part P-1002, the 'internal gear support' (which is part of the planetary gear pack) when changing gears, so again it needs to be rotated to fit.

- Sliging gear.jpg (34.84 KiB) Viewed 634 times
I apologise for being a 'broken record' but my advice is to strip down the whole axle and check, clean repair and reassemble it.
Kind regards
Adrian