65 barracuda vibration

is it your rear or front? I had a cracked K steering gear mount and the front end rattled like a comedy grocery cart wheel. Scared the crap out of me.
You can't have a ball and trunion out of phase., they only mount one way (yoke and rollers are in same plane) at 10mph, your driveshaft should be almost level with just a slight nose down at the differential. IF the rear is shaking badly, I would suspect the spool is fighting either an out of alignment rear or possibly a tire diameter difference, maybe psi is low? Spools are tough on the street.