trying to find source of vibration

Photos?

Not familiar. I would guess? That somewhere "in there" was a ball and trunion U joint. Was it repaired, replaced, replaced with something else?

Does the two--piece have a spline slip joint? You have to rotationally clock those,

You sure it's drivetrain and not wheels / tires?

How 'bout rear wheel bearinngs?

What have you done, IE wiggle test, looking at rotational wheel wobble, stuff like that?