The paragraph below best describes the noise from the differential. How do I go about replacing the bearing or bearings

Whirring noise only while decelerating at any or all speeds is most likely caused by bad pinion bearings or loose pinion bearing preload, and almost never by bad ring and pinion gears

Thanks John

can do it but most of time can go to a auto salavage yard an d get a whole rear end cheapers and just as easy to replace whole rear end

sure its not transmission or gear oil low

It's a 1983 vehicle. You're best off to keep the oil level full and just drive until it goes. Then replace it with a used unit from an auto recycler if you still want to keep the vehicle running.