I am posting this for feedback on this issue that I am having with my 2004 F-150 4x4 with 3.73 gears and a limited slip rear axle. I have over time felt a couple of small shutters after backing out of the garage and moving forward from a stopped position. This has also been felt from a stop sign or light where I would come to a stop and then proceed to remove my foot from the brake pedal and move forward. I have suspected my brakes where starting to hang and thought it may have come from the paint on the calipers so I dove into pulling the rear brakes apart… my emergency brake shoes were toast so I replaced the shoes and hardware. I greased the point where the shoes would touch the backing plate, used caliper grease on the pins and the surface where the brake pads would slide. After adjustments on the shoes I took off for a test drive and the shutter was still there… bummer!
One of the younger mechanics at a garage I go to told me about a known issue (not to me) of the clutches in the limited slip axle. He advise me to add a small bottle of Motorcraft Additive Friction Modifier… well, after adding the 4 oz bottle and driving in a figure 8 pattern for 3 or 4 minutes no more shutter.
I am not aware of any problems with the limited slip and not sure if this has actually solved the problem, but the first 50 miles says it’s a good possibly.
Any thoughts or feedback is welcome… Is this a known issue???

 

I described the same issue at my local dealer on an '03. They sold a clutch pack replacement job. It did solve my problem though.

 

Just a note: After adding the friction additive to my differential I noticed an increase in my gas mileage: by about 1 mile per gallon! True!

 

I think Ford recommends a bottle of their friction modifier with the Trac Lok differentials. Glad that cured it.

 

Yup, fixed mine

 

My 04 has a B6 – 3.73 LS... Is that the same as a Trac Lok differential? I am not familiar with that term.

 

Yes, that is what Ford calls their limited slip differentials.

 

Thanks!

 

My 07 has similar symptoms. Should I just add the friction modifier or change the fluid too?

 

Add 4 oz of the friction modifier then do some figure 8's in a parking lot for a few minutes.

 

That is what I did: The small bottle was only $10 at a local garage that I deal with, so I parked on an incline, crawled under the truck with a 3/8 ratchet and small extension, pulled out the plug (little WD 40 first) and added the 4 oz bottle. About a dozen figure 8's at a low speed and presto, no more shutters... I am also due up for the 60,000 mile service which changes out the differential lube so I will purchase another bottle of the friction modifier and add it when symptoms reappear.

 

I had the same issue a while back. Bought the truck used with 68k on it and it worked fine. I hauled my 27 ft enclosed trailer for a weekend and noticed the stutter, vibration off a start into a turn. I changed the fluid and it helped some but it didn't totally go away. I added another bottle of modifier a week later and after about 500 miles, it is totally gone. 7k miles later it is still fine. Some of the dealers said for sure I needed the clutch pack as the modifier would not fix it. Chalk another one up for a reason to avoid Ford Service shops unless your on warranty

 

Sorry to bring up an old thread, but my truck is having similar symptoms. Is there any chance of hurting something if I add friction modifier to my differential?

 

No. It comes in a bottle with Motorcraft on it; it's made for Ford products. When I had my 60,000 mile service done, I inquired about the issue that I had and they told me they automatically added it with my differential change, which was not due up at 60,000 but I had it done anyways.
It's a cheap fix, so try it first...

 

I have the same shutter when pulling away from a stop. It only happens when I come to a stop sign, and make a right or left turn. This sounds like my problem, but it kind of feels like the front end shuttering, but i guess it could be the rear. I have no idea what gears or what kind of diff I have in my rear end. Its shuttered ever since I bought it last march. I changed the front hubs thinking it was the 4wd, but the shutter was still there. could this possibly be the problem?