[wrenchmen]

Dont use an Impact Gun, use hand tools and work it back and forth.

[GT182]

Use the PB Blaster and let it sit a few days to work in. Once you get them out and replace them, use Never Seize or Permatex Anti-Seize Lube(both are the same). You won't have that problem again if you do.

[Joel Koontz]

Lots of good suggestions.

I have found heat/wax or heat/PB Blaster to work well.

Also agree with the suggestion to hit head of stud with hammer as this can help loosen the threads.

I have found that an Impact Gun does work well, but requires extreme caution in use.
If you turn down the torque, reverse direction often, and frequently apply more penetrant I have found it will often work better than a wrench. The IMPACTS seem to help dislodge the rust with less torque being applied to the stud and hence less chance of breaking the stud off. It can also help because it does not apply side loading to the stud the way a wrench can if not used properly.

BUT let me repeat that the Impact Gun requires extreme caution, and if used recklessly can easily break the stud off.