[68is4me]

I will jump on this. Since you took the rear half of the drive shaft off and vibration was still their, now take the front half off and see if it goes away.

I had an old square with a 350 with a fresh rebuild. The Crank shaft literally broke right in half on one of the mains. So instead of rebuilding the whole thing again, I had another crank laying around and installed it. Worked great till I drove it down the road and had a very strong vibration above 2500 rpms. I checked into the crank that I put into it and found out it was from a 305.

So my point is maybe it is farther up the driveline.