I don't understand the question. What is a backax?? Are you talking about the tailshaft?, rear axle? Rear main engine seal?
Well, There is an axle shaft seal(near the brake backing plate), and a pinion seal(where the driveshaft connects to the differential) If the oil leak is not severe, I would leave it alone. I've never done either job. You may want to consult a chassis manual. They are available on CD ROM. http://www.gearheadcafe.com/buick_shop_manual.html