yea, they came with leather. at least the newer fx4s did. alot of guys swap out seats from an exploder or a stang. i'm guessing from your name, you've got a 98. i dunno if they had leather back then or not.
fine, i'm stock as far as tires go. my milage is better then a 4:10 and i'm happy with that. when i plan to go bigger, it's not too much bigger so i probly won't need to swap it out. i'll loose some low end but i'm not too concerned.
If you're an extended cab, you can swap in 95-01 Explorer bucket seats. They were offered in a couple colors and trim levels. They're a bigger seat by a little, so if you're a bigger person you might want to go another route than a seat swap.