69 Barracuda Clutch Shake Help Needed.

I did check early on to make sure the dowels were present in the block. Those are there.

I haven't checked the rear gear or tranny lubes yet. That's part of my next check, pulling the rear gears. I'm just having trouble wrapping my mind around a tranny or rear gear issue with the car running great down the highway with no hums, clunks or funny noises.