Working on a 2001 F150. Drained the rear diff to replace axle seals. Has the 9.5 inch ring gear with "positraction". The tag on the diff says to use 75w-140 synthetic fluid. My question is... do I need to add friction modifyer?
A ford limited slip does require friction modifiers but some gear lubes have it already (it'll say so on the bottle). I used a redline gear oil in my 95 and worked great with no modifications-- no chattering or other strange behavior.
I have an 03 F150 , 6 cyl. that had a vibration when I turned from a dead stop. This drove me crazy because I could not pin down the cause because it only happened when I turned from a dead stop. A Ford Tech online advised me to empty and refilled with the friction modifier added. He recommended after that to take it out and do some figure eights. That was 65K miles ago and I hope to put on another 100K. Good luck, I hope this works for you.