If anyone is thinking of making a juggernaut to tank, think again; this class is severely lacking in the threat department, whilst single target is a pain to try to hold threat, multi-target is a complete mess and will have you running around like a headless chicken frantically trying to round up mobs.
Until a patch fixes this, Juggernaut's are not viable to tank Flashpoints at high levels. It seems as if Bioware missed something here; if you give a class heavy armour, it does not mean that he will hold aggro. Who knew?
Avoid like the plague.