Haynes has one that covers J-trucks.
Chiltons only covers Wagoneers.
They are both rather basic.
If you're really gonna dig into it, go to eBay and try to score a factory manual in the '80-86 year range that will cover all the FSJ's in great detail.
Chris, I use both Chilton's and Haynes. There's also a website http://www.oljeep.com/
that has electric, vacuum, etc on the site that you can print off. Has some manuals and TSM's that are a great source of info. Hope this helps.
I've been using Haynes for years. You'll finally get to a point where they won't help anymore and you'll have to use the net. Mine helped in the the beginning though. I'm close to Asheville if you need help sometime.