Around here it's almost impossible to sell a used 2wd truck. We have mountains and snow. Nobody wants them, you don't see them, and dealers don't carry them.
I really can't imagine it makes that much difference in fuel economy. When you need it you need it.
It's fairly common for certain options to not be available on certain trim levels. There may be no way around it. It never hurts to ask I guess.
I added power locks to my Chevy crew cab with an $80 kit. I pulled out all the base stereo gear and added good amps and speakers and subs. It all cost way less than what GM wants for their factory ****.