Volcom is correct. It is simple trigonometry. Multiply the sine of the angle by 100% and you'll get the effective dampening. For instance, the image Volcom posted, 100% * sin(80-degrees) = 98.48% (80-degrees is 10-degrees less than 90-degrees). Interestingly, that image is incorrect for all angles except for 10%. I could be mistaken, but here is what it should be:
10% 98.48%
20% 93.97%
30% 86.60%
40% 76.60%
50% 64.28%
So the dampening effectiveness will determine how much body roll the truck will have, as well as how it will ride over bumps. Lower effectiveness = lower dampening = more body roll.
Now in terms of flex, what you want to do is figure out exactly how much flex you have, and run the shortest shock possible at the smallest angle possible. If you have a lot of flex, chances are you may need a 12" or 14" shock. As coloradocrawler mentions, if you run these tall shocks vertical, they will likely protrude through your bed. So you have to lay them down at some angle in order to fit them.
The trigonometry also makes it a cinch to determine how much vertical movement the shock will have depending on the angle it is mounted at. Using the same trig. function, if you have a 12" shock that is mounted at 90-degrees, then the amount of vertical movement will be 12" * sin(90-degrees) = 12", which is expected since the shock is just going straight up and straight down at a 90-degree angle. But if you mount this same 12" shock at a 50-degree angle, then the amount of vertical movement (maximum vertical distance between lower and upper shock mounts) will be 12" * sin(50) = 9.19".
Now you can measure what shock you need and at what angle to mount it depending on how much vertical flex your suspension can manage. Don't worry about the articulation of the axle, just worry about the absolute vertical measurement between the shock mounts on each side. It is also likely that you may have more flex on one side than the other, but it's standard practice to mount both shocks at the same angle to provide a neutral or balanced handling feel on road.
So ideally you want the shocks mounted at the smallest angle from 90-degrees as possible to maximize the dampening of the shock (shock absorbers are designed to absorb shocks after all), so find the shortest shock you need and mount it at the most vertical angle you can with out 1) going through the bed, and 2) running out of vertical shock extension.
BigMike
ps. If you noticed, all you have to do is multiply the effective dampening by the shock length to get it's vertical movement
ie. 12" * .766 = 9.19
