This all depends if you want it pernament 4wd or you would like the option of 2wd / 4wd.
Firstly, I have heard that st185 gt4 gearbox internals can be used to strengthen the AE95's system.
Secondly, You can remove the vacuum control unit and either A - use a lock bar so that your 4wd is constant or B this seems to be easy and cheap and reliable, use 2 central locking solenoids on a 3mm sheet of alloy, create the plate to sit flush with both solenoids on it and have some overhang to "fold" & "bend" it for heat proofing and extra water proofing.
Hope that helped a bit
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