Colin,
Since you are bent on pulling the driveline anyway, do yourself a favor and rotate the u joint while the front section of shaft is removed. There should be no catches in it except maybe at the centered position. If you feel any sort of catch anywhere off center. the driveline needs to be rebuilt. MB went to staked in joints like everone else. You'll have to send it to a rebuilder. Don't even try to do it yourself. The service I use is excellent and fairly priced. They include boxing and shipping in their price.
Power Train Ind. (800) 798-4585

I still think you might have a wheel balance problem or possibly an expanding belt on one of the tires. Swap front to back. I can't diagnose much closer considering the car I'm trying to diagnose is 3000 miles away.

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