Best Penetrant to Remove Differential U-Bolts?

Any the above. PB Blaster, Liquid Wrench, or Kroil. Then time, and patience. Brush the exposed threads with a small wire brush first. Allow the penetrant to soak for a day. When you put your socket on ti, try tightening first but only 1/16 of a turn. Then loosen but do not try to remove all at once if it gets so hard to turn it feels like it will break. Work the nut back and forth until it comes off easily.

Some folks just cut and replace all the U bolts, 'cause doing all 8 like this can take some time.