Pulsing vibration on the freeway

[Mrturner1]

Hey guys,

So I have a 1970 C10, 350/400th and stock rearend. When I'm on the freeway (or anytime I'm driving over 50-55mph) there's this vibration that starts off real slow, as in, it vibrates for a second or two sometimes a little longer and then stops for a second or two, then vibrates, then stops and so on. As I pick up speed and get around 60-65mph the vibration is faster, as in, it vibrates for a second then stops for a second, then vibrates for a second, you get the point.

What could be causing this? I thought driveline angle or something like that, but the angle is fine. The truck isn't lowered or lifted at all. Any ideas?