Engine Vibration at highway rpm/speed

stock 8-3/4 rear end. 489 third member case with 7260 pinion yoke. stock pinion snubber. stock trans mount and engine mounts. not sure how I would check pinion angle nor how I would adjust it with stock setup.

the rear end was replaced by my father and uncle. it came from a valiant or something. When my dad bought the car it had the smaller rear end...7 something. so they found this 8-3/4 at a junk yard, pulled it from that car, and stuck it in the dart. I believe the vibration was there before this switch out and the build sheet on the car IIRC says 8-3/4 rearend.

I did put new leaf spring bushings in a few years back...nothing out of the norm there though.

how do I check and then adjust pinion angle?