have you guys tried not going down the tank tree, and go down the force tree instead for the crit sweeps? this is working for me to keep threat on multiple mobs.
I've looked at the tank tree, but I just don't see much reward from pumping all those points into it.
I think this is kind of a screwy solution to the OP's problem, and the only conclusion you can draw is that the tank tree is badly designed.
I think maybe if guard was changed so that it transferred a lot of threat from the guard target that would work too...but I'd still think tanking in force tree is better than tank tree.