Is the tolerance in the "slip" tight enough to prevent vibrations?? 
There's a reason for splines.
Gnarls. 
Balancing and proper assembly of yolks into tubing prevents vibrations, splines are for transmission of torque over a greater area than would be presented by other methods whilst still allowing the joint to slide.
Technically the square is a spline...
With the hubs unlocked in 2wd, then the shaft wont spin which, in my minds eye, if it cant spin it cant be out of balance. When it is in 4wd its likely to be bouncing limiter so balance and vibration go out the window amirite
