NO and YES.
I did this on my old 4runner, talking 20 year old 4 runner, and I noticed NO decrease in MPG. That is an urban myth in mechanics. Your truck engine is pushing several thousand pounds.... and that front half shaft is designed to not drag.
As for wear. Sure. It will reduce wear. Honestly... after having done this.... you won't see any serious gain. If you wanna do it, go for it.. but my personal opinion is you give up on the fly shifting for 4wd which is a better gain.