Duster Driveshaft length causing Vibration?

THX all for the feedback (Yes my terminology was poor in that I was referring to the "Slip Yoke" in my original thread notes. Was typing it late at night & my brain was fuzzy.....)
Regarding the alignment btwn. the rear axle pinion and trans. centerline: when I measured my original set-up, my trans. output centerline was down ~2.5 to 3-deg compared to the driveshaft and the rear axle pinion was nearly dead-level with the driveshaft. As part of troubleshooting my vibration issue, some friends and i installed a set of 2-deg wedges on the leaf spring perches so that the rear axle pinion is now at ~2-deg upward compared to the driveshaft. This work made basically ZERO difference in the vibration at 55mph. (Not sure if it really matters, but my car also has what appears to be non-stock leaf springs...they only have 4 leafs from the axle forward with the 3 bottom leafs supporting the rear of the car. The car does sit a little low in the rear and feels "stiff" riding in the rear to me.)