With the pull-out drum lock lever pulled out and rotated to the detent slots that keep it out the drum should be able to be rotated by hand. Even if the clutch lever is engaged the drum will rotate a few degrees either direction then hit a stop which is the inherent free play the clutch drive dogs on the end of the drum. With the clutch lever disengaged the drum should free rotate to allow pulling out the cable. I can say that the lever on our winch moves quite hard. You cannot do it by hand so we carry a 24" piece of appropriate diameter conduit tubing that fits over the lever to shift it with. Just use good sense when doing this! A person who has done a lot of mechanical work will have a good idea how hard to pull/push on the pipe to prevent damage. The lever can move a bit and not fully engage because the drive dogs on the end of the drum are not in position to engage. Try rotating the drum around a bit until they do engage and the lever goes to the full engaged position. You can try putting your favorite kind of rusty bolt loosening spray on the lever rod where it goes into the back of the winch housing. We also have other winches you can shift by hand, both 10,000 and 20,000, but they take a bit of effort. Short of totally disassembling the deuce winch to fix the hard shifting problem we will just continue to shift it with the pipe as long as it works that way and keep some loosen-it spray on the shaft to see if it eventually comes around to easier shifting. We have 'bigger fish to fry' than working on that right now since it is working!
If nothing including trying a leverage pipe moves it then disassembly is your only option. Hopefully oil has been kept in that end of the winch housing so rust has not happened.
It is a very good idea to periodically, twice a year at least, engage the winch PTO on the winch and let it run for 5 minutes or so without the drum clutch engaged. Just watch that you do not overheat the brake. This will circulate the oil on the gears and bearings and into the drum bearings on the shaft that goes from the gear box through the drum to the opposite side where the clutch dogs are. Oil will not get there due to level but needs to be stirred by gear rotation to make it happen.
