Automatic hubs unlock in 2WD. So unlocked auto hubs or unlocked manual hubs is exactly like a 2WD, the front axles/diff and driveshaft just sit there and do nothing. With ADD, the front axles turn in 2WD, but the diff and front driveshaft do not.

 

It depends on if you're talking auto hubs vs. the A.D.D. If you're talking about running an A.D.D. system, the hubs are always attached. What happens when you pull the transfer case lever is a switch engages something called VSVs (vacuum switching valves), which allow a vacuum line connected to an actuator at your front differential to engage a sleeve. This sleeve connects your driver side front axle to the front diff. When you switch to 2WD, the another VSV actuates and slides this sleeve off, essentially unhooking your driver side axle. Your passenger side stays engaged, but because your front differential is "open", it only rotates one side of your front gears. However, both CV joints continue to spin.

If you convert from an A.D.D. system to manual hubs (Aisin or Warns are common), engaging the hubs plus the transfer case will engage 4WD. If you disengage your hubs, you'll still have power to your front diff, but no 4WD. If you leave your front hubs locked, but disengage 4WD, you will be rotating your front axle, but will have nothing else. On the plus side, you would be able to unlock your hubs and engage 4WD low, meaning you'd have rear wheel drive, low range. Not sure why you would, but you could.

Now, some of us have chosen to eliminate the whole VSV system for simplicity and ease of maintenance. More valves and vacuum lines usually means more to go wrong. Or, in my case, after swapping to a bigger engine, I didn't want to be bothered with rerouting lines, so I simply yanked all the VSV stuff out and connected a vacuum line to the actuator on my front diff, leaving it locked in all the time. To engage 4WD, I leave my hubs locked and simply switch the lever in and out as need be. If the roads are dry for any length of time, I leave them disengaged. I notice zero difference in fuel economy, but I do notice a bit more of a "drag" while driving. Mostly it feels like there is more mass turning, which there is.

http://www.4x4wire.com/toyota/tech/add/
https://www.yotatech.com/~corey/tech.../matt_hubs.htm
http://www.4x4wire.com/toyota/tech/autohub/

 

i keep my ADD because you will never break a drive flange... and its shift on the fly when I am going up the pass to go skiing. even AISIN hubs break

 

Auto hubs suck b/c they can unlock in reverse, I hate them.

Manual hubs work great b/c you can control when you want them locked. Very little can go wrong and you can rebuild them if they break or buy new ones. Engage one side of the front drivetrain so you can turn if you have a locker. If you got a locker up front you shut off the hubs and no more locker, you want 2wd hi/lo you dont lock the hubs (usefull on rocks sometimes.)

ADD isnt bad but has more than can go wrong..

Overall I have manual hubs on my T100 and love them. The mpg stuff is bs imho.