Hey guys. Just thought I would post this. I have a 04 screw FX4. I had an issue with some chatter in my rear diff. Any time i would move forward from a dead stop and slightly turn I would get a chatter sound from the clutch pack in the rear diff. I also would get a pretty pronounced howling sound at arounf 40-50mph under light load.

Today I replaced the rear end fluid with motorcraft 75w-140 full synthetic gear oil and added a bottle of the friction modifier XL-3 motorcraft. Worked like a champ. No more chatter or howl. Nice and smooth. Just thought I would throw this out there. Later