Why not a 2 speed or quick change rear end option

I grew up with 50s trucks w/2spd rears and I've also repaired them. I preferred the Eatons. US Gear (add-on over or under drive) copied the Eaton design. I like the aux. box or 2 spd rear idea WAY better than just locking out the top trans gears. As mentioned - all the trans gears are still available AND the trans gears are put closer together when the aux. box or rear end are in the lower ratio. All ideas for the dream hauler I plan on building someday. Craig