Just went through a real trial with fixing a vibration that turned out to be the driveshaft on a 96 F350 long bed crew cab so it is a 2 piece shaft.

Vibration had been getting worse for some time...changed the center support, no difference. Replaced the U-joints, no difference. Spline joint good.
Took it to one driveshaft shop that was stumped...told me I just needed to replace the whole thing.
Took it to another one- they took off most of the factory weights, took off the rubber damped heavy steel ring on the pinion end and welded on a washer for a balancing weight on the front shaft.
Smooth as can be now..just can't understand how it can get unbalanced in the first place!