1983 720 4wd -replacing rear axel frame mounted rubber stops

[jeelocked]

On the under side of the frame above the rear axel are rubber impact absorbers with bolts into the frame. Problem. They don't want to come out so I can install new rubbers. One actually broke off. Tried release agent - no effect. There is a 3/4" what appears to be knockout plug into the frame side that would allow getting thread release agent inside the frame onto the threads of the mounting bolts.
If this fails whats the secret to getting those bolts out?
Suggestions?
Is there some safety tech employed on these bolts to prevent unintended loosening? That would seem over kill.
Always somethin'.