Our tod is full time...unless your in 4lo.
Gas savings will not be that substantial...others have disconnected their tod and said if there was any savings it was minimal. You still have the same weight, same aerodynamics and as your tires roll your still turning the cv's, shafts, gears etc in the front end. I have manual hubs, drive daily with them unlocked and have not noticed much in the way of mpg changes...but it does save a little wear and tear on the front end components.
And if it is a ToD disconnect you are looking to do there are several threads on here about that. It has and can be done.