Mission accomplished - dropped the skid and slipped out the bolts with very little cursing!
It was actually very simple. No bolts were stuck (pre-soaked) and the jack opened up the leaf enough to reach the new shackle.
The bushings looked cracked and I thought I had to replace them, but no, they were fine and still plenty to go. This is on a 93 I6. I assume that the dry desert air is good for fasteners and bushings.
As for our friend MDEnce, I know you weren't looking for credit, but credit is due - certainly not for inventing this method
but at least for sharing it.
I think it should be added to the FAQ, since a lot of questions go up about those rear shackle bolts. Instead of starting a whole new message there, I notice that Sentinal02 has editing rights on his message of where he lists different kinds of lift, including the shackle lift. I'll PM him and see if he wants to add the three methods of dealing with the rear bolts.