5 speeds have steel housing and diff carrier. I've heard some later model autos have them, too (maybe 2003 and up...I'm not sure). You'll have to double check that by looking for rust or using a magnet. You can always swap in steel parts if you have the $$$.
As you can probably tell from this forum, Suzukis are good offroaders. They take a lot more work, though. The aftermarket is very slow and sometimes non-existent. Stock ground clearance is sh!#, so you'll need to lift. This forum gives you info on skids. There is a conversion if you want better gearing, manual hubs, and the ability to install lockers (involves swapping parts with a specific make of Suzuki Sidekick). That's what I'm wanting to do, but it's quite expensive, unless you can do your own work. Calmini rockcrawler gears may work on an auto tranny (they don't work on V6 Manuals), but you'd have to call them to check.
Overall, the GVs are fun little trucks if you have the time and money to build them up. Good luck!!
Dave