2 or 1 Piece driveshaft why????

[Pyrotechnic]

I have a one piece in my long bed (the light duty trucks came with them) and at around 100+ MPH I get a pretty bad vibration, and i'm pretty sure it's the drive shaft whipping.

Now that it's due for some new u joints, I'm looking at having the original 2 piece shaft redone and balanced with new u joints and a carrier bearing. 2 short shafts are much stronger than 1 long shaft. I just used the 1 piece because I already had it, and it was freshly redone with new u joints and balanced so I thought I'd give it a try.