Don't discount the prop being out of dynamic balance. Our '71 Cessna
172 had a peculiar vibration from about 2250-2350 that would shake the
instruments in the panel and you could feel it in the floorboards and
thru the yokes. It was smooth as glass between 2000-2200 and also
above 2400. Getting the prop dynamically balanced all but eliminated
the vibration. It's still there, but vastly diminished now and very
tolerable. We only got charged about $150 to put a $1 AN bolt and nut
thru a hole in the flywheel for the balance weight :-) I wish the
vibration could have been completely eliminated, but the A&P who did
the balance says that with the particular prop we have, a Sensenich
74DM7S14-0-58 that was installed along with a new engine a few years
before we bought the plane, that it is common to have some minor
vibration in the neighborhood of 2300 rpms.