Re: 1973 Mercury 50hp- stuck in drive?
I agree with BR. If the shift cable and linkage are moving, pull the lower unit... look on the port (left) side of the motor (standing behind the boat) with the wraparound cover taken off of the block... Two cables should come to that side from the front of the boat. The bottom cable/linkage should move back and forth when the controls are moved forward and reverse. If this is happening, check the propeller in forward, neutral, and reverse to see that it spins in opposite directions freely in forward and reverse, and that it spins in both directions freely in neutral.
My guess is that it will spin freely the same direction no matter where you shift the controls. In that case, pull the lower unit (the bottom of the motor, where the propeller is).
Should be a 5/8 nut on either side of the LU... these don't come off completely until you get all the rest off... There's another one on the underside (another 5/8 if like my 69)... and, again, if like mine, a 3/8 hex bolt that is accessed from a hole (look down from the back of the motor) using an allen (hex) wrench. I recommend a T-Handle 3/8... Lowes/Auto parts/hardware store.
Once the 3/8 hex is out, the "fin" above the prop should drop down, and up in there is a 9/16 nut to take off. After all those are off, go back to the 5/8 nuts on either side... and start loosening them further, which will push the lower unit down.
If you have help, have a buddy wiggle the prop and/or flywheel (on top of the outboard) while you separate the LU. Might wanna have a wooden block beneath it because it is heavy when it drops if you're not ready for it.
Do all of this in Neutral on the controls. Once you get the LU off, you will see a small shaft/pinion (other than the huge drive shaft) that sticks up a few inches from the LU. Wrap a rag around it a few times and then take some channel-locks (pliers) and gently rotate the shaft one "click" at a time, then check the prop to see that it freely spins in both directions. Keep clicking until it does so.
Now both the controls and the LU are in Neutral. Carefully put her back together (keep checking the prop for neutral as you re-assemble) and try the gears again!