What size tire are you running?
if not too big, you could go for a "lunchbox locker" like an EZ-locker...
just pull out the 3rd member and replace the spider gears.
ARB's are sweet, giving you open/locked choice, but they are $$$
For max beef, you could get a Detroit, but the install can get spendy as well.
or throw a spool in there or weld it up!