how about the constant velocity joint and the u-joints? also you can narrow
it down by removing drive shafts. also can you feel it under your feet?
also make sure all the u-joints are in line. another thing is are you sure
you don't have a broken spring in the clutch, or a bad throw out bearing? I
have had them bad right out of the box. It also could be the cincro out of
a line just a bit in the tranny. Just a hair out of sink can make a world
of difference.