There are "kits" available from for all FJ80's (both FJ80s and FJZ80s) to convert to part-time 4wd. You DO NOT have to swap out the transfer case to convert to part time 4wd with the "kits".
As I said a couple of the guys who have done a "conversion" of types (the manual hubs and removal of front drive shaft when not off-roading) have gotten as high as 17.8, up from an average of about 12. If you consider 15 mpg as an average and $2.25 as an average for the price of gas, that's a savings of nearly $400 over a year (for 10k miles and over $450 for 12.5k miles).
Aisin manual hubs are somewhere between $50-80/set. . . it's a significant savings especially with the price of gas nowadays. Over a few years, that's quite a bit of money. Even if you went the kit route, you'd still start saving money after the first year (kit is about $500).