Vibration on Highway at 60+

[ynst]

Thank you all for your great responses!

Yesterday we ended up swapping the driveshaft from my father’s 71 RR. Back on the highway and the vibration was still there, but did seem to improve it some. It vibrated from 63-68 MPH but seemed to get better at 70 and above. If at 70 and putting into neutral and letting it coast to 65, it doesn’t vibrate. However if I raise the RPMs in neutral in that speed range, it does vibrate.

I’ve ordered the rear bearing and will give that a shot next. I’ve also seen some folks with the tail shaft weight dampener installed. Do you think it’s worth throwing one of those on as well?

Thanks again and will let you guys know how it goes!

However if I raise the RPMs in neutral in that speed range, it does vibrate.
It ain't the drive-line. It's the engine that's causing the vibration.

Time to balance the engine rotating assembly.

My engine balancers (3 of the 4 I've used have passed away) all say the same thing. There's a trick to getting rid of that persistent engine vibration around 3000 rpm but nobody pays attention to it.
My late friend Goodwin Hannaford up in Maine got rid of that vibration in my GTX a couple of years after I got it back on the road. The vibration had been there for years but, back then, I didn't think anything of it, especially since I was drag racing it every summer and the lift bars were always banging the rubber snubbers on the front spring hanger all the time.

"Plymouth is out to run you over this year" instead of win you over this year like the commercial said.

By the way, those are the same wheels and hubcaps that are currently, still,on the car.

That's me blowing the doors off my kid brother's SS 396 '67 Camaro at Lebanon Valley Dragway

The car was off the road and in indoor storage for about 40 years before I got around to putting it back on the road.

I noticed the vibration right away having been driving newer, smoother-running cars for years.
It never bothered me back when the car was new but after all of those years driving "comfortable" cars I really noticed it after I got done repainting it and putting it on the road.

Since there was a bit of rust on the cylinder walls from being stored all those years and the rust had pitted a couple of cylinder walls I decided, after putting two or three thousand miles on it after the repaint, I'd have Goodwin go through the engine for me at 113,000 miles back in 2017 and the vibration went away. The only minor vibration I'm sensing is tire balance, now.