I'll be honest you picked a heck of a job to try and do on a lift. If you can I would personally put the sterndrive nuts back on, and drive/tow the boat to shore where you can get it on a trailer.
If you can not... The sterndrive is sticking for 1 of 3 reasons.
1. Engine is out of alignment, pushing down on the input shaft, causing it to pinch.
2. The sterndrive input shaft is rusted to the gimbal bearing, this happens then the bellow tear and the boat is left to sit.
3. The stenrdrives input shaft is rusted to he engine coupler.
Those are the only 3 possibilities. f you want to try doing this on a lift what you would need to do is get a few 4x4's and some through bolts and make yourself a beam you can throw on top of the boat lift beams horizontially. From there you can attach a come-a-long to the beam. Loosen up the motor mounts on the engine. And then try lifting up on the engine a little bit, especially the rear of it. Sometimes that will do it. If it wont do it then the shaft is rusted to either the coupler or gimbal bearing (or both) and if that is the case you have to get in behind the engine with a sawzall and cut the input shaft and then try pulling. If it still won't come out then it is rusted to the gimbal bearing, and you would remove the engine and then hammer out what is left of the input shaft to drive the sterndrive and gimbal bearing out.
I've done this quite a few times, it is a bit of work.
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