U-joints run at angles will vibrate. If you look at one cap, it will speed up and slow down as the shaft rotates.
If you set the operating angle between the t-case output and driveshaft equal to the the operating angle between the pinion and driveshaft the vibrations of the two u-joints will cancel out and you won't feel any vibration from the shaft. That is, in theory...
In the four wheeling world, that doesn't work that well. IIRC Spicer said the operating angle on its joints should be under 3 degrees to insure no vibration.
Most of you running lifted trucks have at least 10 degrees at each joint.
The theory is that if you run the same operating angle at the t-case joint and the pinion joint that you will have no vibrations. In the real world that is not the case, because you are exceeding the angle limitations of that setup (not binding, just from a vibration standpoint).
Some people can make it work and get little/no vibration on a lifted truck.
I would say that with more than 4" of lift you have basically no chance of having no vibrations without a CV shaft.
You have a pickup with a pretty long rear shaft, and 10 deg. isn't very much IMO. I bet you would be okay with a standard driveshaft, but a CV driveshaft would eliminate all doubt and would run smooth.
I know that I typed a lot of info here, but I would probably run a standard (non-cv) shaft if I was you.
