Strange vibration at 60 MPH and above.

[The Swed]

Some facts, my truck has a rebuilt 350 and rebuilt TH350, rims are straight and balanced both brake drums in front are new/replaced. The vibration starts at 60 MPH and the tac reads 3000 RPM and 3500 at 70 MPH. I have disengaged the transmission at 70 and the vibration is still there I also put the transmission in 1st and ran it all the way to 4000 RPM with no vibration so I believe it’s mostly likely the drive train? So U joints, drive shafts, or that axle bearing? What are your thoughts? Thanks in advance for your inputs!