You cant. They are meant to be lubricated by the rear differential oil. You don't need to lube them. They're a sealed bearing unit. Also, there is no "rear differential" on a 1994 Pontiac Bonneville, or any other car that is front-wheel drive. In fact, the '94 Bonneville has independent rear wheel suspension, meaning that, other than the frame/body of the car, there is no connection whatsoever between the wheels. Even with standard suspension for a front-wheel drive vehicle, the rear wheels typically have only a bar, rod or frame between them, and rotate on their own spindles, independently of the other. If you are having problems with your rear wheels, jack the car up, take the wheel and brake drum off (if you have drum brakes) and rotate the hub by hand. See if you can feel any catches, or rough places in the rotation. Even better, get a mechanic's stethoscope. It has a long metal rod instead of that round bell that a doctor's has. Take the wheel off, etc. and rotate the hub while you listen to the backside of the hub. You may be able to hear what it's doing. Hope this helps.