bad vibration

1995 Z28 Camaro convertible, V8, 4L60E tranny, everything else stock.

A few months ago, I installed Kooks headers, Yank SS3200 stall torque
converter, and an FLT (Level 3) auto tranny to a standard rubber
tranny
mount. Just prior to that, the only other things I've done is add a
Meziere
electric pump, ignition wires, plugs, MSD cap/rotor, new Yokohama
tires
all around.

A couple of months ago, I replaced the broken stock rubber tranny
mount
with a poly mount and noticed an increase in vibration. But in the
last
month, or say, 1,000 miles, I've noticed a fairly "violent" vibration.
I've
rotated the driveshaft 180 degrees with no improvement.

(yes, I realize that more vibrations are expected with a poly tranny
mount,
but these vibrations are out of the ordinary).

The vibration occurs mostly between 1,800-2,000 RPM, and seems to be
most apparent in 3rd gear - if i manually shift through the gears,
it's not
so noticeable in 1st or 2nd, but is most noticeable when i shift into
D. In
fourth, (circle D), it's not as apparent.

I have also noticed one other thing. Let's say I'm running at 50+ MPH
and
above 1500 RPM and I let off the gas. When the RPM's hit 1500, the
tranny
downshifts to 3rd VERY noticeably (never did this before). Also, if I
mash
on the gas (say, from a stop) conservatively, the tranny shifts into
overdrive
prematurely, at around 1500 RPM at 45 MPH, which it never did before.

So here's my question - and I'm mostly interested in determining the
vibration problem now. What is the best method to determine where the
vibration is coming from? Could it be the flywheel, harmonic balancer,
torque converter, tranny, or driveshaft components? How can I reliably
eliminate where the problem could be coming from?