I have a lower unit that the shift shaft spins. It worked perfectly 1 day ago and it hasn't done anything but lay on my shop shelf when I changed the impeller!

The Story:
I have a 1974 Mercury 850 (85hp) motor that I purchased with a bad lower unit. I went Friday and picked up another lower unit off of a 1979 Mercury 90hp motor. Yes, I am now aware of the gear ratio differences but that isn't my problem. When I went to pick it up it shifted perfectly through F-N-R by small clicks on the shift shaft. I brought it home, replaced the impeller and housing, go to install it and the shift shaft has some drag but just spins about 180 degrees. Doesn't change the direction at all. It appears to be stuck in Forward (turning the shaft clockwise from the top). I was scared to mess it up so I didn't force anything but if you turn the shift shaft counterclockwise it hits a hard spot, if you turn it clockwise it hits what feels like a tension load and if you press against it the prop stops spinning as if in Neutral but it doesn't click into place. I can't for the life of me imagine what on earth happened to it!

Any help would be appreciated.

Josh

Keep turning it, it will go 360 degrees, when it gets hard to turn, turn it until it "clicks" it will then be back in gear . one more click will be neutral and 1 more will be the other gear, then it will spin 360 ish degrees again.
So, turn it until it clicks, then turn it 1 more click and it will be in neutral. then install the lower.

Clockwise or Counter clockwise? If I go counter clockwise it hits what feels like a dead metal clunk. Clockwise is the way that seems like it would keep going if I PUSHED it. Make sense?

Sorry, clockwise

Thank you so much. Should I be turning the shaft or prop while I am doing this or do I just crank it around?. Sorry, I'm just worried about messing this up. These things don't grow on trees & I only have the knowledge you guys hook me up with

Trust me. If I had the extra $ this sucker would be at a Marina. I guess necessity is the beginning of knowledge acquisition

You should be able to just turn the shift shaft, if it hangs up turn the prop shaft (either direction)

Copy. I'll try it. Thanks again.