Roadblock: Can't remove top nut holding rear shock (2002 XG350L)

I developed a "thunking" noise on the rear passenger side and narrowed it to the cup or upper spring pad. I removed the strut assembly and compressed the spring. I attempted to remove the upper nut holding the shock with a wrench and a vice-lock, as is usually done. The gripping area above the nut broke off, however.

I need to remove the shock. At this point, I'm fine with destroying the current shock and replacing the rear pair. They look like original equipment which means 50K and 18 years!! So basically, I'm looking for advice on removing these corroded in old shocks.

 

I have heard removing the top nut on the strut requires an impact wrench
But, I am not about giving advice about dangerous situations.
Could not take the idea of anything wrong happening because of my bad advice

 

Success!! I had to use an angle grinder with a cutting wheel to cut off the shock nut. On one strut assembly (the one making the "thunk") the foam rubber bushing was totally rotted away and I could cut the shock absorber's shaft directly. On the other, I used a grinding wheel to get down to the washer. For some unknown reason, Hyundai used foam rubber for the shock mounting bushings. I'm surprised they didn't give out long ago. The industry standard, and the OEM Hyundai replacements are made of firm, solid rubber.

Now I'm hoping to tackle the hopefully easier issue of some noise I've narrowed to the power steering belt.