twopeanuts- my $.02
Hey looks like it has come your time to deal with probably the most common problem I've seen on the rabbits (and even later models). I've had this issue now on all 3 vw's I've owned with varing results. Sounds like right now you have the classic single-crack-skipping gear. Alot of times they'll crack in two and fall right off the shaft, leaving you with no odo action. This was the case with my first car. At the time, I was able to go into a Tacoma, wa area speedometer shop, and purchase the gear for a couple bucks. I simply pressed the new gear on; however, it only ran for about a year and then cracked again!

I also had bad luck with the odo on my wifes old 90 jetta, but with that the digits started moving out of place and out of order.
My pickup is the only vw I've got now, and has given me the best luck. I almost couldn't believe that the odo was still working when I bought it, with the extra diesel vibrations and all. I think it was about a year or so into ownership when the gear cracked and started doing the tell-tale skipping. :cry: So I remembered where 5 years prior I had gotten the gear at the speedo shop, but this time they wouldn't sell it to me. (But they would rebuild my speedo for me for $100+). No thanks. Something about the ethical implications of people messing with thier own speedometers/ odometers. I didn't quite get that, since I was just trying to make it run so that it would have an accurate milage. Anyhow I was left with repair since I didn't know where else to get a new one.
I ended up just pulling the gear off, and taking a strand of copper out of the middle of an electrical wire and wrapping that copper strand around each side flange of the gear to give it some extra strength and prevent it from stretching and skipping on every revolution. I secured the copper strand and the gear back on the shaft with common craft glue. I kinda figured it would be just a temporary fix until I could find a new gear, but it has been working great for over 5 years now so...?
I think there may still be places that sell the gear new online, and I'm sure if you find a good speedo shop they will sell you one. My friend just had the same problem on his old volvo (also a vdo gauge) and a specialty volvo parts place was able to fix him up with the proper gear (unfortunatly not the same gear as used in the vw's). If you were going to get a new gear I'd do like roger and probably drill it out slightly and epoxy it on the shaft or something because I think they are engineered a bit tight and that contributes to thier failure, but purely a guess.
The RogerBrown link posted above is a great one and describes what I'd guess is your problem.
Hope this helps, let us know if you run into any troubles.