Yes, you need a selectable locker. But for the price, you almost might as well go for a T-Case swap and an automatic locker.
Two choices that I know of:
OX--Cable Activated--Probably $699 (+ shipping if applicable) (+install of around 5-8 hours labor. I think they might be charging like an extra $100 for the cable too now on top of that, but not sure...
ARB Air locker--Similar cost once locker, compressor, and install are bought and completed. Maybe just a little bit cheaper.
You could do a T-case swap and an auto locker for like under a grand still too.
I opted for the OX and trash-loc together make it go combo...
6", 35's, Detroit/OX, L10, 4.56, Warn 9500