Need Advice, Suggestions

john, i think those rubber encapsulated driveshafts were done back in the day to help absorb driveline vibrations. fast forward 46 years and i can see where the rubber can get hard, or possibly even break loose like an old harmonic damper will do. combine that with the pinion angle issue and that might very well be the source of the vibration. i agree with most everything you said regarding the springs, but i'm not sure how having them switched side for side would contribute to the vibration. nevertheless they need to be switched around from the way it sounds.
did the body guys put those springs on for you?