Engine Vibration at highway rpm/speed

It does not do it in neutral or Park when slowly revving motor throughout the rpm band.

It only does it when driving and only at highway speeds. 60-75mph.

Oscillation is def coming from the floor and trans tunnel. Trans and engine both have brand new mounts. It is def mounted tight, but I need to retorque since remounting the crossmember and driving 100 miles or so now.

Brake drums and shoes are brand new. Back right does rub a bit on the backing plate, but is wearing down now and the vibration was there when the old original bell shaped drums were on anyways.

Taking out driveshaft and running the trans/motor in drive at that rpm and speed may help eliminate a few things... but I agree I may spew trans fluid from rear seal on trans since the yoke would be missing. I guess I could take my old yoke from the old driveshaft off just to plug the trans. I guess that test would eliminate driveshaft rearward if the vibration was still there. If the vibration goes away that tells me pinion angle or something driveshaft back is the culprit and driveshaft is brand new. So my bet at that point would be pinion angle since the vibration is in the tunnel directly down the middle of the car. Also the third member is brand new as well. I’m suspecting that when my dad put the 8-3/4 rear axle in from the junk yard... they may not have thought about checking the pinion angle.

Thanks for the app suggestion. I’m going to research this pinion angle so I can check it out. New rims and tires will be on tomorrow so I can eliminate the originals as the problem as well.

Thanks for the suggestions guys! I really want to track this thing down and fix it. I feel like if it was in the motor that it would run badly and would do it when revving in neutral. So I guess crank and rods/pistons must be okay.

The pinion angle looks okay to me... but 4 degrees would be hard for me to see with my eyes.

Also.... magically lost my Speedo when driving it few days ago.. haven’t looked that over yet... but my guess is snapped speedo cable. I need to spin it with a drill to see if it still works. Surely it isn’t the speedo gear on the trans... that was replaced to compensate for the change to 3.73 gears.