Here is a good article discussing the strength of 5.71's:
http://www.gearinstalls.com/410suck.htm
Also I am wondering, is that corrosion in there? You guys using a diff breather extension? Swamp water and/or mud getting drawn into the third through the stock diff breather could cause hell in any diff. That pinion looks mangled.
The carrier does look a little beat. I'd take the 4.10 third you have and like you said rebuild it with 5.71's. That locker may be rebuildable too.
Good luck to you.