1) is it possible to fit into '91 HZJ80 front FF axle e-locking diff from later model ?
2) Is it possible to exchange rear axle from later model (>'94, with lockers) into '91 ?
3) can I put some aftermarket air-conditioning into '91 HZJ80 (1HZ engine)?
I don't know about the lockers from later models, but I do know that air con was a dealer fit option on the lower spec models here in Australia. This being the case it shouldn't be too hard to retrofit, if you can source the bits somewhere.
hdj81s came with f/r lockers and full floating front/rear axles for 1990 and 1991 so the answer to one and two is yes it can be done but you likely don't have the ECM to control them. the locker part requires that you will need to buy the locked axles from somewhere and custom wire it. there are threads here on how to do it on a vehicle that is already FF rear. where you are going to a FF rear it might require other brake changes, and a complete rear axle out to the lug bolts.
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2. Yes you can! I just did it over the Christmas holiday. I put in a 96 axel. I had to go to rear disk brakes. If you can find a 94 or later axel it will bolt right up. The 96 sway bar is heavier on the rear but the body mount points are the same. You will need a new e- brake cable too. I had a minor problem piloting the drive shaft to the differential flange that required me to grind on the pinon nut a little. Otherwise it was a bolt right up.
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If you can find a 94 or later axel it will bolt right up. The 96 sway bar is heavier on the rear but the body mount points are the same. You will need a new e- brake cable too. I had a minor problem piloting the drive shaft to the differential flange that required me to grind on the pinon nut a little. Otherwise it was a bolt right up. 
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