I am wondering where the 4wd disconnects. If It is right in the transfercase, or at the front diff, or both?

The reason why I am asking is because I will be running a smaller spare tire. On my old truck I would just disconnect the hubs and run the spare on the front. If I run different size tires on the front of my 3rd gen, will the front diff always be turning?

I have a '99 standard 4runner, with the real transfercase, no 4wd buttons for me!

 

smaller spare tire? what did you do with the regular tire??

 

on my 97 the front drive shaft is disconnected in 2wd. Newer center diff/awd ones might be different.

 

In your 99 the transfer case disconnects the front shaft in 2wd and the ADD disconnects one side of the diff. Both front axle shafts will still turn, because you don't have hubs, but it won't do any harm for them to turn at different speeds as they won't be connected together.

 

Is the ADD a special option, or does it happen automaticly when shifted into 2wd?

 

I guess I could just jack up the front end, spin one tire, and see what the other one does.

 

ADD has been standard since 89 or so, unless you have manual hubs. It basically splits one front axle in 2 with a sliding hub and collar. In 4wd it engages to link the front wheels to the diff, and in 2wd it is disengaged. In yours I think it's vacuum operated but 01 and up use an electric system.

More info
http://www.off-road.com/toyota/tech/add/