Best Penetrant to Remove Differential U-Bolts?

Demonic said
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.

I agree. Personally, I prefer Mopar Heater Control Solvent. It was made for heat risers, I think, but it'll work on anything rusty. Heat was mentioned, and that can help. I'd also suggest an impact gun w/ 6pt impact socket, but like the quoted post, work the nut gradually back and forth carefully. Be patient. It is usually worth trying to save the factory u-bolts, because aftermarket bolts are not flattened at the u-bend and are not friendly to the housing tube like the oem bolts.