I cant remember what the cut off is for sure...but here are some things to think about
If you are thinking about a full case locker (ARB, Detroit, OX, etc) you are gonna wanna pay someone to install them, due to dealing with re-setting the gears. You pay $250 (est.) in labor to get your locker installed ($500 in labor if you do both axles at the same time). Now...6 months...a year...down the road you are ready for new gears. On top of the cost of your gears, install kits, etc...you now have another $500 in labor that you have to pay again. If you do it all at once, most people will charge you the same labor as doing one or the other. Just something to consider, but as far as the cutoff...I honestly cant remember so i cant answer your specific locker question.
If you can save the money and do it all in one shot, it will save you some bills in the long run.