i'm wondering if not having the yolk splined for the first inch or so, is to allow ease of assembly. in other words, the unsplined section allows the yolk to be supported on the tailshaft, while the assembler then has it easier to rotate the shaft to engage the tailshaft and yolk splines.
however, the tailshaft only has a certain length of splines on it, so [in my opinion] the yolk must [or should] have the female spline length that would be completely engaged with the male spline length on the tailshaft during all suspension movements and articulations.
i also agree about getting the shaft trued and balanced for high speed operation, not for truck use. this is especially true if one has an overdrive transmission and deep rear gears.