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.
To change the rear axle on a 2004 Buick Rendezvous, first ensure the vehicle is securely lifted and supported on jack stands. Remove the rear wheels and brake components to access the axle assembly. Disconnect the axle from the differential and any suspension components, then slide the axle out of its housing. Install the new axle by reversing the disassembly steps, ensuring all connections and components are properly secured.
The fill plug is on the rear cover of the differential. Remove the rear cover to drain the fluid.
buick roadmaster 1992 rear axle does it have limited slip diff
On the rear axle?
The 2004 Buick Regal has 57.1 in. of rear shoulder room.
The 2004 Buick Regal has 53.3 in. of rear hip room.
The 2004 Buick Regal has 36.9 in. of rear leg room.
The 2004 Buick Regal has 37.4 in. of rear head room.
Top of the rear axleTop of the rear axle
The rear track of the 2004 Buick Regal is 5 ft. 1.3 in. (61.3 in.).
It is in the center of the rear of the axle.
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.