I'll give it a shot. When the x-fer case and the driveshaft are on a straight line to each other, the u-joint works like it's all one part. (Heck, you wouldn't NEED a u-joint, just bolt 'er up... [img]images/graemlins/grin.gif[/img]) But when they're NOT on a straight line, you've got two things (output yoke and driveshaft yoke) that each want to spin in their own respective orbits. You put a u-joint in between them to kind of join the two. This doesn't work perfectly, because in order to make it work the u-joint has to twist. Here's the hard-to-get part. Let's say you're looking at the u-joint straight from the x-fer case output shaft while it's turning. Two ends of the u-joint (we'll say north and south) are turning in a perfect circle, because they are attached to the yoke on the x-fer case. However, the east and west ones (from your perspective)lok like they are turning in an ellipse, because they are in an orbit that is skewed from yours. In fact, as far as the power is concerned, it IS going in an ellipse. This makes the east-west axis actually change speed during the rotation, going fast-slow-fast-slow in each full turn. Consequently.the DRIVESHAFT ITSELF is also going fast-slow-fast-slow, and that means vibrations for your Jeep. Can't have that, can we? So, to cancel out the fast-slow-fast-slow, we have to have the same thing on the differential side, only at an equal but inverse angle in order to slow down the fast parts and speed up the slow parts. This equal but inverse angle makes the diff pinion and x-fer case pinion parallel.
Now, if you got all that, you can see how a CV joint works also. It's got two u-joints in it, so think of the little double yoke in between the u-joints as the world's smallest driveshaft. it's the only fast-slow-fast-slow part, so the actual SHAFT of the driveshaft is turning at an even speed. That's why you bolt up a diff in a straight line to the driveshaft on a CV setup; you're already at smooth power, why mess it up?
That's my best shot, hope it helps. If it doesn't, check out http://www.4xshaft.com/
and look in the "driveshaft geometry 101" article in the tech info section. I personally think it should be a 400-level course; it's pretty tough stuff to get your head around. [img]images/graemlins/smile.gif[/img]