No advice, but a comment  :>)

I've got a similar problem with a 91 GL.  Moderate vibration from 40-50 mph.
I tried balancing all wheels.  No effect.  The tires needed replacing, so
I've got
4 new Michelin MXT's (stock size - 205/70 r14).  No effect.  Some months
ago I thought I read a post where someone found it was a clutch/pressure
plate problem.  However, wouldn't that make the vibrations sensitive to
RPM's, rather than speed?  My vibes are there in the same speed range
whether I'm in 3rd or 4th.  I may try a 4 wheel alignment next.  I'm also
wondering if there's an out-of-round wheel.

At 11:26 AM 3/18/1999 -0800, you wrote:
>Dear List Members:
>
>My patient is an '86 Syncro Weekender.  Symptoms:  bad vibrations
>
>        1) Strong vibration at middle RPMs at 50 mph.
>           The car shakes so badly that I think that
>           ancillary problems may occur.
>
>        2) The vibration happens ONLY under load, but
>           it seems especially strong under a light load.
>           Otherwise the car drives as smoothly as silk.
>
>        3) With the hatch up, I have noticed that the
>           engine vibrates strongly when accelerated
>           into the middle RPM range by hand (sitting
>           in the garage in neurtral.)
>
>My fixit efforts:
>
>        1) New tires all round. Rotate wheels.  No effect.
>           (Rear tires were badly worn.)
>
>        2) New rear wheel bearings, and brakes.  All new
>           rear CV joints. (One rear bearing was worn out
>           and one rear brake tended to lock on.)  No
>           change, but better brakes.
>
>        3) Replaced driveshaft with another used driveshaft.
>           No difference.
>
>        4) Replaced front diff with another used front diff.
>           No change.
>
>        5) Currently replacing engine mounts, guessing that
>           the old ones were worn out.
>
>If #5 (engine mounts) doesn't work my remaining guesses are:
>
>        1) Engine misses at certain RPMs and I need to have
>           the engine examined.  Could this shake the car?
>
>        2) Transmission.  Is this RPM dependant shaking under
>           load a common symptom of a bad transmission?
>
>Since the engine already seeps slightly at the heads, and since
>the remaining possible solutions I am looking at (engine, tranny)
>are very expensive, I'm feeling the need for advice ...
>
>I'd really appreciate comments.
>
>John
>jmates@ichips.intel.com
>
>
>--
>\ o /  __o        __|    \/
>  |     /|   __\o    \o   |
> / \   |\  /)  |     ( \ /o\
>
> John Mates, Intel Corp.
>
> JFT
> 5200 NE Elam Young Parkway
> Hillsboro, OR 97124-6497
>
> jmates@ichips.intel.com
> (503) 264-8023
> #include <std.disclaimer>