Ron,
Can almost follow that, BUT, as usual, it does not address the case we have on P6s, and many other vehicles, of UJs installed out of phase by design, and to quote Julius sumner miller " why is it so?". I have searched the web for an explanation to no avail. When I got my car I pulled the prop shaft (forget why)and checked the alignment of the arrows, and corrected the alignment. coincidentally it had a 2000 part no. Over a few 100 kms I had no issues, but on one occasion I had a quite severe vibration, at low speeds, only lasting a km or so. No idea what happened - foreign matter in a wheel?