Maybe Keith or Robbie will chime in here, but my understanding is that Robbie was not able to get either driveshaft (stock or DC) to run smoothly without vibrations. Robbie installed the new Slee front control arms (which are beefy and very cool), but they were really designed for the caster needed by the six inch springs. Keith ordered the 4 inch springs, but the Slee shop told him the front arms should also be compatible with the 4 inch lift.
So, the likely problem is that the arms are giving the truck too much caster and/or the DC shaft is defective.
Now, Mikey, is that a better story for you?
Evan
P.S. By the way, I have an older DC front drivshaft from Slee. The u-joint at the front of the shaft was already going bad (I'm thinking it was a Spicer unit), but Robbie installed a new Toyota OEM (Koyo?) u-joint in its place. So that's a good thing.