I took my '01 F150 with 3.55 L/S Diff, into the dealer yesterday to have them check out the rearend chatter / grinding noise that has been getting worse since it was serviced with 75W140 a few months ago. The wife has also noticed it, so it must be bad.

The technician " verified the noise, serviced diff with XL3 friction modifier, drive 200 - 250 miles"

I told the service technician, I had already added friction modifier to it, after they failed to do so when I had it previously serviced. (53K miles)

I was looking up XL3 F/M on the internet, and ran across a Motorcraft Service Chart for 1997-2004 vehicles. It calls for XY-80W90QL, 80W90 Prem Rear Axle Fluid, not the 75W140 that is in our warranty books that came with our trucks.

XL3 F/M, EST-M2C118-A, Additive F/M (Low Odor) is different from C8AZ-19B546-A, EST-M2C118-A Additive F/M that is called for in our warranty books.

Has anyone come across this information before, and could this be the reason we are having rearend problems with our trucks?
Servicing with the incorrect fluid requirement and incorrect F/M?