Birdhunter, this comes up often, and they are right, Search works well for this.
Lunchbox lockers (Which is what we call them because they go inside the carrier, not replace it) work well, but not everyone appreciates their road manners. They work well Off-Road, and are inexpensive compared to a locker which replaced the entire carrier and may or not be selectable.
The draw back is that they are not a geared differential; they either power both wheels, or in a "Coast Mode", when you apply no power, the wheel moving the fastest is allowed to over-run the slower one (like in a turn). When you apply power again, the locker will re-lock and tie the two axles together.
This means that on the road, you need to modify how you deal with turns, or sudden acceleration; smooth entry into turns with no power, and if you have a manual, smooth shift will become your game.
I have heard that the "No-Slip" lockers are better than the regular lockers because they don't release or engage as suddenly.
-Ron