[Accelo]

It is not always clear why the OEM's go with a one or two piece driveline as the engineering is interpretive. And cost is always an issue.
In general terms, a two piece is used when extra length is required.
Or when the vehicle cross-members do not allow large diameter drivelines.

The simple explanation is drivelines tend to whip at higher speeds.
To combat this tendency they get bigger in diameter as they get longer.
This is the reason front drive-lines are much smaller in diameter as it has little to do with load capacity.

Phasing of the driveshaft is done when the driveshaft is welded up. If yours isn't marked from the factory and it can be assembled in as many different different locations as it has splines all is not lost.
The The U-joint phasing dictates the driveline go together in one of two ways. Set the phasing correctly and drive it. They rotate it 180 degrees and drive it again.
If it affects the truck you will likely know which is correct. In general. if your issue is caused by the driveline balance it would have to be a mile out of balance to impart the energy to the truck you are concerned with.

Balance is always at the rotating speed of the mechanical part. Drive-line will cause a vibration issue at 3.7 times the rotation speed of the tires. (If your axle ratio is 3;70) Maybe you can tell the difference by feel? I would also put the vehicle in neutral and coast at that 40mph. It's possible the vibration will go away. If it does it's in the motor.
Including moving the tires from the front to the rear. Jack up the rear and observe the tires as they rotate. Sometimes you can see a bump that isn't visible otherwise.
Eliminate as many issues as possible. One can learn a lot this way.
Hope this helps.
Rick