Tough nuts??

[Accelo]

A lot of good advice. I will rehash some of it.

Use some cutting oil and a die-nut to clean up the threads. Down side is the large NF die nut is expensive. If the threads are damaged cut the U-bolt off flush at the bottom of the nut with a cut of saw or Saws-all.

I typically heat the nut to red hot using a Oxy Act tanks. Not with a cutting torch but using a rosebud. Although a Torch works well, just not as well because you are looking for a temperature differential and the faster you heat it the better off you will be.

Impacts have improved so much over the years I recommend using one. Reason is they will work fast, before the heat dissipates.

I have never had this fail me. Before driving the U-bolt(s) out double nut the end of the u-bolt threads. This way you will not damage them.

If the trailing arms are not swollen, between the two half's, with rust you have a good chance the bolt will be reusable. Inspect the bolt after removal for defects. I have never had to replace them, but I don't live where they salt the roads.
Cheers