I have the same problem with my 360. I also went thru changing out the tourque converter several times (to tighten up the stall speed mostly) and hoped to fix the vibration after doing a B&M weighted plate and zero balance torque converter from Shaun @ "whatever that top converter companies name is"
I finally had the drive shaft balanced and new u-joints and that appears to help a LITTLE bit but still not completely.

What is interesting also I find it only seems to vibrate under load. Once the converter kicks in and under load (pedal to the gas) and SEEMs to get worse the higher the load.

I am also thinking my problem is with the damper as that is where I am going to work on next.

good luck.