Interesting thread. I too, went to replace the shocks on the wife's XJ last year, only to break all 4 bolts off trying to get the rear shocks off. PB blaster did not help, and they weren't THAT rusted to begin with.
I went ahead and broke all 4 off and removed the shocks. I then took it to a good fab shop where a brave young man with a welder cleaned the areas with a rotary grinder, then welded together several large washers and grade 8 bolts together, then welded them upside-down (bolt sticking down) to the frame. It was a permanant fix and cost me about $75 total.
When I reinstalled those shocks, I smeared plenty of cosmoline (heavy, heavy grease) on the bolts and nuts before assembly so that it will never sieze again. The shock mounts are now even stronger than before.