IMHO, for your mild wants, if you are only running 31"s on 8" rims don't change the diffs, just change the transfer case gears, drop in at lease one lockright locker, drop in some 3" lift springs/shocks with a drop pitman arm and hit the trails. You can do this in about two weekends all solo. It's all bolt on. This was my first setup.
Again, the only option is maybe keep the springs and just put them above the axles. This requires welding and may give more lift than needed for the 31"s.
Why are you choosing 31"s for those trails?