A gas tank for a passenger vehicle is usually quite flimsy and surrounded the frame to protect it. Saddle tanks on a highway rig are usually made of heavier material because they are more likely to get hit. So even though OEMs can still build saddle tank setups, that doesn't mean its legal to retrofit onto a light vehicle that didn't originally have it. If there are no inspections in your state, then it may not matter.
However, as you just mentioned, the underside would not be well protected, and since you have a 4x4, that could be the most vulnerable area if you ever get high-ended near the edge of a trail.
I get that you want to use a freebie that just dropped in your lap, but overall, it would be a lot less headache to just replace the stock tank. They can be had on fleabay for anywhere between $100-$200 depending on whether you go for the cheaper steel tank or fork out extra for a plastic tank.