You can see a few pictures of a 722.1 transmission in this thread - this is the four speed version of your three speed 722.0
http://www.peachparts.com/shopforum/diesel-discussion/304903-722-118-automatic-transmission-rebuild-monster-diy.html
The parking mechanism is shown on the first page - though the pictures aren't really made for your problem!
I have read that people try and remove the nut on the output shaft of the transmission by engaging the transmission in park and then having a go at it. This might be a bit of problem if you need to do this. I reckon this kind of makes it a transmission out job as it'll be a bit of problem stopping the output shaft from turning when it is fitted to the car unless there's a way of stopping that shaft from turning with the transmission fitted to the vehicle...
...I don't know if it is possible - perhaps someone here has done it?
Anyway - nuts and covers and all the rest of it shouldn't be considered until you are sure that the linkage isn't working properly. Disconnect the shifter linkage at the transmission - turn the selector and check to see if the park pawl engages when it isn't connected to the rest of the linkage.