Welcome aboard.
Do you know if any work has been done on the lower unit - Like impeller replacement? If the lower isn't installed properly, the shift shaft, etc. can easily be out of alignment.
One thing you can do is have someone move the throttle lever on the control while you look at the shift cable to see if it moves on the port side of the motor and the shifting mechanicals move with the cable. If they do seem to move correctly, it's time to remove the lower unit and check shifting there.
No need to consider rebuilding the lower unit yet.
