Sounds like something’s screwed up in the linkage or cable then, if the shift fork was contacting the bump ring and it’s not coming out of gear, you’ll often feel the shift lever pulse as it rides up and down the ring

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Tried to pull the gearbox off and I am stuck. I can’t see a clip to disconnect the gear shift cable. Maybe it is the reason I am stuck in reverse.

Push the drive back up to normal position, drain the fluid down and remove the back cap, then you can disconnect the link from the cable that connects to the lever that attaches to the shift shaft, then you should be able to get it off

Is normal position, normal when running (level) or normal when towing (up)?

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Took the rear cap off and used a large screwdriver to move the shift lever in order to disconnect the cable. No luck, it is quite stuck. Is that greeny white grease normal, or could it be water mixing with the oil? Any further suggestions to get it unstuck?

There’s a Allen screw recessed in the lever arm where the detent ball rides, remove that and see if the shifter moves, you can also remove the cotter pin from the shift link and disconnect them, then you should be able to remove the drive , the green grease is normal

It is summer and boating season here but we have had torrential rain with flooding.

Anyway, the weather is great again and I got to the boat.

Removed the shift cable and the shift lever. Slid the drive back.

Drive still stuck in reverse. I guess now we know it is in the drive.

What to do next?

Couple options
Find a drive shop and tell them you want it rebuilt.
Buy a new drive
Buy a used drive

Okay, thanks for the help.