Johnson 1978 55hp, No Reverse

I have a 1978 55hp Seahorse with a hydro-mechanical gearcase. The engine runs in neutral and shifts into forward with no problem, but it will not catch in reverse. I?ve adjusted the shift cable and even tried to manually shift the linkage with the cable disconnected. The dog clutch tries to engage and the prop will slowly spin, but I hear the gears clicking (slipping) and I feel the cable harshly vibrating, which tells me it will not fully engage. My thoughts at this point are the that the dog clutch or reverse gear is badly worn, but could this be an issue with the plunger or something else?

Unfortunately I don?t live near the boat, this is going to be a weekend project for me, so I?m trying to prepare myself and gather all possible parts that I may need. I?ve always had great success searching this forum for every other problem I?ve had, but couldn?t much information on this. I appreciate any help, thanks.