At 04:56 PM 12/2/96 -0600, you wrote:
>I have a 1990 Synchro Westie.  During the last month it has developed a 
>loud, low hum and strong vibrations.  They get worse with higher 
>speeds, but seem worst between 40-50 mph, and above 70mph.  Engine rpm 
>does not affect.  They are also getting worse/louder with time as I 
>have not been able to locate and fix the problem.  Distinct, strong 
>vibrations are transmitted to the entire van, but seem especially acute 
>in the stickshift arm.  Moving the arm to certain positions seems to 
>quiet and change vibrations a little.
>
>I suspect, in order, the viscous coupling, driveshaft, rear 
>differential, and transmission.  Cursory check of (rear) bearings and 
>CV joints seem fine.  Any experience or suggestions?
>
>Richard Lee at pcpros1@ix.netcom.com
>
>

I'd pull the driveshaft out and drive it, not to tough to do. The bus will
still go and you can see if the vibration is still there. This will at least
cut down on your troubleshooting.
You have crawled under and given everything a good shake haven't you? Maybe
something is loose?

Good luck
Rob