Mike,
You are on the right track! Same problem on my '94 GLS about 3 yrs back. As I recall, (forgive me if I am using the wrong names for these parts) the tumbler fits into a round cylinder maybe 2 inches long. This part is made of what I regard as pot metal. On the inside end of this cylinder is a hook or connection the ties into the central locking system. On mine, it was this hook-like area that had broken. Therefore, while the key would operate the drivers door but no longer controlled the central lock system or the windows/sun roof, I think. The cylinder and some washers cost my under $20 (three years ago) at the local dealer. As for doing the work yourself, IF you are a handy sort of guy, sure you can. I picked up a HAYDEN manual at Pep Boys for $15 (I know you guys prefer BENTLEY'S) This manual had fairly good step-by-step procedure regarding removal and replacement of the components. B4 you start, read and re-read the procedure. I think it involved opening the inside panels of the door too. Be _real_ careful when you remove the lock tumbler, there are little springs and parts that will jump out, run under the car and hide. (don't ask how I know) Laying out a beach towel under the door may save you some grief.
Good Luck! Dexter in AZ