if it has the F series IRS rear end then u shouldnt need a diff upgrade..
to find out the type of diff u have check the ID plate under the bonnet and look for the "trans axle" code.. that will tell u the type of diff u have.. the "W5#" type code will be the gearbox and the "F283" type code will tell u the type of diff (ie F series) which is found in your early model supras and are pretty strong (250+ HP i think... can anyone confirm???)