Hello,
I'm sorry to hear they're making you replace the axle. Will the Belgium DMV consider information showing your axle is not cast iron? The center of the housing is cast steel, while the tubes are made of mild steel. If you clean the paint off the tubes you will likely be able to see where the ends have been welded on by the manufacturer. I may be able to dig up some technical information from GM or from or an axle rebuilder that will verify this.
You're looking at a Guv-Lock centrifugally actuated positive traction differential (GM Option Code G80). If the axles are turning at significantly different speeds, a spring loaded weight will engage linkage that causes friction clutches to engage, providing traction to both tires. These are not durable positraction differentials and will most likely fail if you are fond of "burning rubber."
Technical stuff begins at 11:07.
https://www.youtube.com/watch?v=EsvWCFi_IPk