And of course bear tanks are crit immune with talents.

I would suggest getting omen - letting your tank get a lead of a few swipes then bringing your team in to batter away.

What spec is your lock? if it's destro it can bang out some big numbers off the bat so waiting a couple of seconds helps your bear get the aggro it needs.

Also - make sure you have soul shatter bound to a key so you can drop the locks threat.

WIth my shaman team I found swipe spam was more than enough to keep and hold aggro - and that was with pvp builds on the shaman that didn't take threat reduction.