Rear diff oil confusion.

[Splorin' 461]

2003 V8 Limited 4x4 w/tow package 170000 miles

For a little while now I've been having this strange drag/vibration in the rear. Some here have said that changing their rear diff fluid helps or solves the problem so I thought I would look into it.
When I bought the X in October I had my local shop do some work. I dug back in the paper work and found that the refilled the rear diff w/ synthetic 75w-90. According to the manual that is wrong so I decided to change it out.

According to the manual the rear axle requires synthetic 75w-140 with 4oz of XL-3 Friction Modifier. So I went to the dealer and bought what is needed.

Just now, I damn near have the drain plug out and I happen to notice "75w-90" embossed in the rear diff case.

So my question is, what oil does this thing take and what would you do?

Thanks!

 

[swshawaii]

See NOTE (Page 2)

LIMITED SLIP AXLE CHATTER, SHUDDER, BINDING SENSATION, OR VIBRATION DURING LOW SPEED TURNING MANEUVERS - REVISED
http://www.revbase.com/BBBMotor/TSb/DownloadPdf?id=132459

FREE TSB's & Wiring Diagrams
http://www.bbbind.com/free_tsb.html

 

[Splorin' 461]

Thanks, that's exactly what I needed to see!

 

[5whiskey]

75w-140 is what was originally mandated, but then Ford tried going to the lighter fluid for fuel mileage. It didn't work out so great for the LS differential. I run the 75w-140 with friction modifier in mine and it cleared the rear end chatter that existed because of whatever fluid was in it when I bought it.