Being a 120Y it can have either a borg warner diff (Plate on back) or a h150 in Australia as they came with either,The Lift out bolt in Diff centre H series is best.

If it is a borg warner then you will need a whole diff assembly and that means ideally a 1200 Ute complete diff,then you can find either a 3.9 or the more common 3.7 out of either a stanza (3.9) or Bluebird series one (3.7) and all of these are the stronger H165 and desireable for long term A 15 Power.