I've managed to break the logjam in my tanking gear that was holding me up from upgrading for the longest time. One of the things that helped me out a lot was the Guardian's Leather Belt, the season 4 rogue belt. Emmerald puts it as the third best tanking belt for bears in the game and unlike all the usual high end PvE gear it comes with some crit immunity. 30 resilience, equivalent to 45 defense for crit immunity.
With the belt I could finally upgrade from the old Vindicator's Leather Boots I was using before. It makes me a bit sad to say it, but I really don't see any way to get a T5/T6 bear suit put together without some sort of resilience gear. I have a Belt of Natural Power which is marginally better than the PvP belt, but I need crit immunity from somewhere.
The Season 4 belts are easy to get; 17,850 honor, 40 AB marks. The S4 boots and bracers are also great but they have arena rating requirements, so you have to be more serious about PvP to get them. For belts consider also the Guardian's Dragonhide Belt; that's the official druid belt and has more armor. Alas, it wastes points on intellect and so has less agility and stamina, but the strength makes it better for DPS in both bear and cat form. I stuck with the rogue belt because it has 4 more resilience, and this piece of gear is all about maximizing my crit immunity.