I really like my original factory fitted cable lockers - their simplicity and I'm impressed that all the cables, levers and links are all working fine after 36 years.
But to retrofit in the US surely expensive and complicated....
How about a Torsen diff ? They are gears only - no air, cables, electrics or clutches. Don't lock up 100% but will work very well in the conditions the OP mentioned above. I've seen them quite often in Landrovers - and they work just quietly fine if you don't do much wheel lifting.
They are just replacing the diff centre - no other parts needed and all the rest can stay stock.
They were sold as Detroit True Trac - are they not available anymore ?
I would stay clear of Detroit Lockers, Lockrite or same systems, (Is the mentioned "Lunchbox" one of those ? ) if you are doing a lot of road driving.