Found this article today, thought it explained some things pretty well. Biased toward leaf springs, but that only changes how you fix angles, not what causes the problems.
http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml#Double-Cardan-Measurements
http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml#Double-Cardan-Measurements