Basically, you want to cover the t-case, cover the transmission, cover the oil pan, and cover the gas tank. Those are the areas to focus on.
You could fab them up yourself, but steel plate is VERY expensive.
Fortunately, I can help 'ya out...
Click here:
http://www.globalsoftware-inc.com/coolerman/forsale/TrannySkid.htm This is coolerman, who's sold his Xterra. He's got custom plates that were made, which are as strong, if not stronger, than the aftermarket ones. And they're dirt cheap, compared to the other prices on the net.
If you don't get them, though, you can at least look at them and get a good idea of what all you need to do, as well as how to mount 'em, etc.