I have heard that this is possible on a FJ80 (91-92) and I think there are even "kits" available to do this. I have also heard that it is not possible to do this on a FZJ80 (93-97). While not possible is probably not entirely accurate, you would probably have to swap out the transfer case for a part time one. Either way though, I doubt that you would ever recoup the cost of conversion through gas savings. I have a 89 FJ62, same engine and transmission as a FJ80 but it is lighter and part time 4wd and I average around 12 mpg -- I doubt that a full time 4wd heavier FJ80 gets much worse.

My suggestion is that if you can't live with the gas mileage of a stock 80 series, you should probably buy something else, sorry.