It depends on the conversion you use. There are actual conversion kits that add a piece to the transfer case/center diff and adds manual hubs in the front. Then when you want 4WD you need to hit the Center Diff Lock switch and turn the hubs. If the Center diff isn't "locked" then no power goes to the front.

Another way would be to add manual hubs to the front and then removing the front drive line. Then you'd need to drive with the center diff locked to get power to the rear wheels. But anytime you want 4WD or AWD you'd need to install front drive line again. With the Hubs unlocked and the center diff locked you'd essentially be driving in 2WD but you'd still be spinning your front drive shaft which would still require some power.

One thing to keep in mind if you didn't already know this is that the FJ80 is a full time AWD not full time 4WD. To get 4WD like your 4Runner you have to lock the center diff. The center diff lock on a FJ80 just puts you into 4WD.