As far as the factory units go they are not bad units. Ford sets them up with rather low preload so they do not grip as well as they could. The preload can be increased thus making them grip much better. But as with all clutch type l/s they wear out with use and become less and less effective. Similar to clutches, the more you use them the quicker they wear out.
As a general rule of thumb you need a specific carrier for a given gear series. i.e. all 3.xx:1 gears will use the same carrier, and all 4.xx:1 gears will use a different carrier, there are exceptions. The axles and axle housing will be the same, so unless something is bent, broken, or cracked there is no need to replace the entire axle housing assembly.
IMO for stock to moderate performance levels the best bang for the buck is the Lock Rite locker. This works inside the stock open carrier making it a locker. So there is no need to reset gear tolerances, or even remove the carrier from the housing. The Lock Rite does not use friction clutches so there is nothing to wear out. It is a little noisy but then most lockers are.
I think calling a gear supplier like was said above is a very good idea.