I can't really answer your question, but I plan on doing the swap in my AD 3/4 ton as soon as I find a 4.11 third member.
I will say that $400 to $500 for a later third member sounds outrageous. Maybe it's your part of the country, but I can get a complete rear end for about $60 at my local u-pullit which has 67 to 72 trucks quite often. I just missed out on getting one because I haven't had a chance to get up there and the truck went to the crusher.
