Your '06 TJ's gas tank skidplate is adequate. Early TJ gas tank skidplates were not, they were only thin sheet metal, but the factory beefed it up significantly sometime in the early 2000's. The only use I'd say a newer skidplate needs replacing for would be some pretty extreme wheeling, like rock crawling.
What you definitely need first is an oil pan skidplate. The oil pan is vulnerable and if you punch a hole in it without you noticing, your engine could be toast. Some prefer to go with an entire engine skid vs. just an oil pan skidplate but which way you go depends on your budget.
Some insist on adding a skidplate to the front steering box, which can't hurt, but in 15 years of wheeling and ten years of rock crawling, nothing has ever touched my steering box.
After the oil pan, I'd next add rocker guards to protect the area under the doors. Rocker guards that actually bolt to the tub are far stronger and preferrable to those that bolt to the frame using long mounting brackets.