Getting parts & supplies together toe place spider gears. 130k miles & driver side gear killed. I will be using Mobil 1 LS75W140. Do I need to use a friction modifier?

 

Only if your diff is a "Limited Slip" rearend. The pumpkin tag should tell you, or it will be on the build sheet. 130K miles is pretty soon for a rear end to go out. If there were mods to the wheels and tires I could understand this OR modifier was not added with the last gear oil change. Good luck