I've had similar vibration from a bad CV joint. If the joint "pits" or binds
from poor lubrication the binding may be felt under engine load as a shaking.
This happened to my '87 while still under warranty.

Tom
Austin, TX

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From:   Vanagon mailing list on behalf of Monte Davrill
Sent:   Tuesday, October 21, 1997 12:34 PM
To:     Vanagon@GERRY.SDSC.EDU
Subject:        55MPH vibration problem...

Hey Vanagang,
Cruzing back from Santa Cruz last Saturday, my '86 developed a vibration
that was hard to figure out. At between 50-55MPH a shaking (roughly the
frequency of the revolving tires) takes place when I'm on the gas but stops
instantly when I let off the pedal. This happens on level roads and hills
and only in the above noted speed ranges.

Any clues?
Thanks,
Monte

P.S. Thanks again Kyle for drilling my new rear bumper for the old impact
strip. It fits great.