before you get carried away with replacing the transmission routine someone already mentioned the inching mechanism.
One common problem these units have had and there is a service bulletin out on this... the inching can affect the drive if that lever/pivot bracket is binding up. (yes the one on top of the transmission the cable hooks to that pushes on the inching spool )
i'd remove the cable and take it off the post and check the o-rings , replace if necessary and lube it up, reassemble it and then recheck. THEN if necessary run pressure checks. IF that lever is binding it will throw your pressure checks off.

Thanks and have a nice day!