i attempted to make a diagram, if i had you here, i could sit you down at my drafting table and show you more throughly.
every rearend uses a means of location, either that being leafsprings, 4 link, 2 link( +panhard for side to side location)
this keeps the rearend in a specified location, you dont want the rear wheels to beable to walk, you want them at a set distance so that your car will travel straight down the road.
this is why you never see a leaf spring car/truck with shackles in the front of the leaf spring, only the rear of the leafs.
this is just 1 part of the equation.
the 2 nd is suspension movement, as you go over bumps, uneven terrain, and torque you want to create the least amount of load on the chassis and occupant.
this is where leafs are the most versatile. you can locate the rearend, and have a set type of ride all in one.
with this flex, according to where you have the rearend located at, creates an arc. your leaf springs, coil springs, and coil overs do there best to keep this arc to a minimum
none the less, there will still be an arc.
with any arc, the lateral dimension will change, granted ever so slightly, but it will change.
a U-jount and thin walled driveshaft will not beable to handle this stress.
so as the rearend goes up and down while your driving down the road, the driveshaft slips in and out to compensate.
hopefully i explained that well enough for ya to understand.
to me its alot easier to look at a diagram, but i could not find my damn chassis books.
i know you still have a question.
"what about the front leaf spring perch and the traction bar creating 2 different kind of arcs?"
yep. thats exactly what its doing. they are working aginst eachother to eliminate the stress on the leafspring, where the leafspring is trying to flex
thus eliminating "wheel hop"