An Armory link would help however with what you're tanking you probably just need more gear/Stamina. 490 is indeed the cap for uncrittable and the priest is completely clueless. If he doesn't know the defense cap he most likely doesn't know how to heal.
If you're dying on multi-mob pulls you need to design your party with a decent amount of CC. Heroic mobs hit hard, dont be discouraged.