Some rearends need one, like the aforementioned Ford 8.8, since due to its design it works as a rock-catcher.
The spring bolts that attach leaf springs to the axle really need attention. Having the bolts hang down below the spring make them quite susceptible to catching on rocks and even getting broken off. At the very least, the bolts should be cut flush with the nuts.
What is the best material for skidplates? Well, steel far outweighs aluminum (pun intended) and is the most common choice in more ways than one. It's easy to weld, cut and form, along with being a fairly inexpensive material. Its major drawback is its weight. Aluminum seems at first to be the ideal material in that it comes in a wide variety of alloys, depending on the application, that offer strength with a much lighter weight. The most common to find and to use is 6061, which offers a good combination of strength, corrosion resistance and machinability. However, it really doesn't like to be bent in a really tight radius and impossible to bend if heat-treated, and it's not as easy to weld as some of the other alloys. Being somewhat softer than steel, it can gouge a whole lot easier, and in some cases this "gouging" may not let it slide over rocks as easily as steel.
Stainless steel, Teflon, HDPE, UHMW, nylon, and Delrin are just some of the other materials that have been used for skidplates. The popular choice with a lot of rockcrawlers is a plastic referred to as UHMW or HDPE which have high impact resistance and a slippery quality. Commercial-quality cutting boards are made from HDPE, and sometimes you can find it close to the dimensions you need from a commercial kitchen supply house. UHMW, while quite a bit more expensive, is six times more abrasion-resistant than steel, has no cold embrittlement problems, and works from minus-155 to 200-plus degrees Fahrenheit. However, you're pretty much limited to using flat sheets of either material. Some people have built their skidplates out of welded steel or aluminum and then covered them with the "plastic" material.
Whatever you use for a skidplate, just remember its function is to not only protect a component but to allow the vehicle to slide over an obstacle and not get hung up on it. Oh, and don't forget to allow for access around the skidplate to things like the oil drain and fill plugs and lube fittings.