driveshaft phasing

[old cool]

Sorry for all the questions on the same day. Hope others can benefit from all the expertise on this forum.
I've read a lot about vibration and drive shaft phasing. As I understand it, the front and rear yoke have to be running the same way as they enter input and output points. Kinda hard to explain, but when looking at front and rear of drive shaft, one yoke CAN'T be running east and west while the other end is turned north and south. I get that, but I don't see how they can be a 180 degrees off if they appear visually correct. Questions:

1.) If yokes look same (mirror each other) can they still be out of phase, and if so, how do I determine?
2.)Can I drop drive shaft rotate just one yoke at 180 degrees?
3.)Would out-of-phase yokes be real shaky during the drive? Can they cause mild vibration? I get milder vibration at 30-35 mph. It is felt in floor gear shift handle and steering wheel. I don't really notice the body, front end, or seat shaking.
4.)Since the slip yoke going into the trans has a blind spline, will that only go in one way? Or, can the blind (missing) spline slide in at any position into the tailshaft?

I pulled in a trans shop and was told there were many variables to consider. I do understand and believe this is correct. But, I'm still gonna try a few things just because the hunt and fix is what we old guys do. Last year I started a thread called drive line noise. The responses were so abundant I didn't no where to begin. Hence, smaller, narrowed down questions is my new approach. Thank you for bearing with me.

As always, I am grateful for help. Love the forum.