Many things can cause a driveline vibration:

Too much downward pinion angle will cause a vibration on deceleration.

Too much upward pinion angle will cause a vibration on acceleration.

Driveshaft run out

More than 1" slip yoke (internal) stand out

Less than 3/4" slip yoke (internal) stand out

Sloppy fitting slip yoke bushing

Sloppy fitting slip yoke splines

Sloppy fitting U-joint saddles/missing clips

Solid motor mounts

Poly transmission mount

Loose pinion yoke

Engine/bellhousing alignment

Torque converter/vibration damper balance

Exhaust grounding out on body

etc...