Other than changing the rear axle oil at the initial 1000 mile inspection service there is no requirement to change rear axle oil on 86-90 models, however, if you have a limited slip differential, I would recommend a change every 30K miles and be sure to add the proper friction modifier additive to avoid clutch chatter.

For open differentials changing the oil every 60K miles is probably a good idea if you expect to drive the car over 150K miles.

Duke