Re: lower gearcase reassembly
Gearcases? Plural or typo?
With the lower unit removed from the engine.... does it shift into both gears and back into neutral as it should?
Did you change the height of the shift rod measurement?
If with the lower unit removed from the engine, it does not shift properly either... something is not assembled properly within the lower unit.
If the lower unit shift rod measurement (a critical measurement) has been changed, you'll need to have the lower unit on the bench to set it properly.