Support barbarians are only more defensive than an offensive barbarian because of their choice of stats on gear and passives, the actual legendary items they pick do not tend directly make them more defensive with the exception of their weapons (as opposed to a support Monk which tends to take abilities and items that give damage reduction and healing). ...
You are supposed to
group monsters with Furious Charge + Ancient Spear;
keep up party buffs - Ignore Pain and War Cry;
help with DPS using Ground Stomp (as DPS classes can use Bane of Trapped or other debuff dependent items) and triggering your Oculus Ring buff.
Comes from your build Skills and Runes page:
The Support Barb ...