I was reading that the 2005 rear differential is much weaker than those used by 2006+ C6's? If this is true are there any recalls on the '05 diff's? Or what can I do to beef this thing up?

Yes, typical GM brain trust. Add 50 hp to your flagship and save $5 by engineering the differential out of thinner material. It's the case that is failure prone. As the pinion attempts to turn the ring gear, it also pushes it outward, breaking the case. In severe instances, it will take the tranny with it. BIG repair bill. The DTE differential brace will save your tranny in case of breakabe, but actually does nothing to prevent the failure. GM attempted to fix the situation with a software change at the last minute. They call it "Torque Management". It's Bull-Pelosi. Instead of fixing the problem, they make the engine and horespower that you paid for non-existant in the lower gears. There are rumors of attempts to make GM recall/replace the differentials with the stronger units from the later cars. But, with the current state of affairs at GM..............doubtfull. I think the drag strip guys usually swap in a C5 differential for it's strength.