At nearly 100k miles there is a good chance balancing wont do it. The u-joints have likely loosened up to the point of needing replacement. One other consideration is the center shaft between the trans and t-case. On the 1990 it is still a u-jointed shaft. There is an updated shaft that uses CV's there and often that is the fix. Have you figured out which shaft is the culprit?

I might persue a balancing first just based on the cost of new shafts. Finding a shop that can work with the odd-ball flanges is tricky, but it can be done. Even if you can get the shafts in Europe, they will still cost you at least $800 each.

If you have some time to wait there is a fellow working up a DIY u-joint replacement kit for the G. It may be ready by spring '05.

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1995 G320
1984 280GE
1971 Unimog 416