Gary,
     I love my TiiCo conversion, and I too have experienced the harmonic
vibrations.  I was wondering if you had heard of anyone solid-mounting the
exhaust system? What I mean is to do away with the rubber mounts that the
muffler support brackets attach to and replace them with some type of solid
spacer.  The muffler will still be isolated from the body by the motor and
transmission mounts.  I have yet to understand why the system is rubber
mounted.  The 2.1 exhaust was solid mounted to the motor, so why can't this
one be mounted solidly as well?   I'm having some spacers made and I am going
to give this a try.
     As for vibrations, I did find a couple that did not have anything to do
with the exhaust.  I had one at 3000-3200, and it ended up being the a/c
compressor bearing.  This was actually more of a buzzing than a vibration,
but you could feel it when you touched the a/c compressor while the engine
was in the 3K rpm range.  I also put the "high density" foam in my motor
mounts as per a suggestion for a vanagon.com contributor.  This helped out as
well.

chris stanford
montgomery, al