Some have a drain plug, others require cover removal.
It is filled through a plug in the cover, or on the side.
Remove rear axle cover, oil will drain out, reseal and install cover, fill fluid.Remove rear axle cover, oil will drain out, reseal and install cover, fill fluid.
there is 2 different ways to change the fluid this is a drain plug on the front of the rear diff or you can take the back of the pumpkin off on the rear diff.
The rear axle.
About 1.25 liters of oil goes in the rear axle of a Ford transit. It takes 75W90 oil when you replace it.
synthetic gear oil
It's the oil/fluid that is in the rear drive axle (and front axle on 4x4).
Axle seals
To change the rear axle fluid you can remove the rear cover from the axle and drain it that way. If you don't want to remove the rear cover you can use a pump and a hose to drain it through the fill hole.
In many cases they are lubricated from the gear oil in the differential.
This depends on what Buick it is. Most do not have oil in the rear "Axle" as most Buicks of this vintage did not have a rear Differential. The ones that did need oil in the differential have a fill plug on the rear cover or the side of the case and this is where the oil is poured in. Follow the instructions in the owners manual to get the correct level.
purple
Vehicles with front wheel drive have a straight rear axle meaning there is no gear housing in the rear axle. Vehicles that are rear wheel drive have a gear box in the middle of the rear axle to change the rotation of a drive shaft to turn the rear axle. If you look under a full size pick up truck rear axle you will see a large gear box (typically referred to as a pumpkin) in the middle area of the axle. That is the gear box differential.