Why put a locker in something that is not going off road? In that 06' you already have a limited slip, what more do you need? 3.07 is low, but 31 are too. But on the flip side, the 06' has a deeper gear ratio in first and second gear range. I don't think you will have a problem with 33's.
As far as the D35, run it til' you break it, I dare ya'. It will handle more than you think. Beleeeive me. I run 35's and trail pretty damn hard, with some pretty sticky tires, on the d35. I will not lie, I am going bigger, but that's me. If all your doing is mall crawling or the everyday beach cruise thing, don't worry yourself. That little time bomb is OK for what your doing.
If you do decide to invest in a new axle, the d44 or a ford 8.8 might be an investment. But, I would not put any money into that d35.
PM me if you have any Q's...