20 armor is kinda low for a tank, though it's hard to get past 30 armor without D3 gear.
Crusaders sit anywhere from 47-54 armor, at best, now (which is 27-34 more damage reduced PER ATTACK; i.e. very substantial).
Might want a 5th player, perhaps a zerker, to help with crowd control/tanking, since your Sin and Sorc are glass canons by comparison.