how did water get in and destroy upper and lower? the new lower is a sealed unit other than transfer areas between the upper and lower.
Ayuh,..... Obviously,... There was a Leak.... It could very well have been in the Upper, as they both share the Oil Cavity..... The Oil, or Water in your case Freely flows between the upper,+ lower...
The 4 times you used it last season is Plenty of time to do the Damage,....
Then You compounded the problem by Not draining,+ refilling Before Winter.....
At this point,... An aftermarket drive from SEI will only set your back about $1300./ $1500. for a Brand New drive with a 3 year warranty...
When I used the boat for the first time late 07 after repair the port side(repaired) was hard to shift. Had to force it,
Ayuh,..... That sounds like the Lower Shift Cable is Junk..... Replace it....