On your comment on Vikings with Shield and 2HW, note the last sentence on page 24.
The Vikings can have both, but cannot employ the shield in any phase that they employ the 2HW. You decide at the beginning of each phase.
As such, against shooting the Vikings will have D6 (assuming no Light Armour). However, in any combat phase they decide to employ the 2HW (*), their D is worsened by 1, so they will have no D in this instance.
(*) There may well be combat phases where the Vikings decide to not use the 2HW, such as when their opponents have no armour, and want the benefit of their shield instead.
Re Dacian / Basternae falxmen, I recall that their are illustrations showing warriors both shielded and unshielded, but I cannot recall any contemporary reliefs showing a shield in use by a warrior using a falx, that is to say a weapon that could be wielded two handed. Several illustration show warriors with shields in the same scene as unshielded falxmen, but you can either not see their weapon or the "falx" in question is a short single handed weapon, not much more than a large knife really. As such, I would suggest that falxmen carry a 2HW and have no D (i.e. unarmoured). Other warriors would, of course, be "typical barbarians", with a hand weapon/spear and shield.