I have finally found the causes of my vibrations in my Westy Syncro TD.
Originally the vibes came on strongest at around 3000 rpm. I crawled under
the van while pulling the throttle cable to the appropriate rpm, and
watched/felt as the skid-plate assembly began to shimmy and shake violently.
I originally thought that attaching the rear of it to the engine carrier
tubes should work well, but that is not the case. Apparently the tubes need
to move as well as the engine mounts! SO, I removed the skidplate assembly,
no more 3000 rpm shake at all! Now a lesser vibe at 2400 rpm...hmmm....again
under the van, thinking "it must be the trans mount!". Nope! Nice and quiet
at all rpms there. In the engine compartment, revving it to 2200~2400, I
noticed the engine carriers shaking.....hmmm......"what if I add a tie bar
between the two carriers, as the original diesel Vanagons had?" Quickly
whipped one out of a piece of scrap, bolted it up, and....NO MORE
VIBRATION!! Very strange! Now all that remains is a very slight vibe that
comes right at 2000 and immediately is gone. Maybe I will balance the engine
when I rebuild it next winter. It definitely is more fun to drive without
those pesky vibrations!

  The other thing I decided to try was enlarging the intake of the (stock
Rabbit TD) airbox. Seat-of-the-pants, it has more power in the upper RPMs!
Guess it needed to breathe more air. Next: an intercooler. AFTER I go play
in the snow of Oregon and Montana! <:-)

Karl
'87 Syncro Westy 1.9TD