My Jeep still has the original 40 year old steel tank in it. Its a California Jeep, so salt and corrosion isn't that big of a deal. You are in Sacramento, so again exterior corrosion shouldn't be a problem. As long as you use the Jeep interior corrosion should not be a problem either. If you want to go with a 21 gallon tank instead of the 15 gallon size, I believe you have to go with the poly tank, but I could be wrong. Anyway, welcome to JeepForum and the insanity known as CJ's.
Be sure to fill out your Jeep's profile so that when you have questions there is a quick reference for others.
By the way, I have spent a lot of time in the Sacramento area. Lived in Woodland for 18 years and worked in Sacramento a lot.