I did some searching of the forums and found out about the clutch pack shudder. I have a couple extra questions I couldn't find answers to.

2004 lifted F150

Bought my truck beginning of this year. Everything was fine. Over the summer, it started doing the rear end shudder on cornering at slow speeds. This winter, in the snow in 4hi it shudders going straight when under acceleration (talking 0-15 mph). According to others, you can try different fluid with a friction modifier to solve the rear end shudder.

If I was going to try that prior to replacing the disc's, would I stay with the 75w90 or switch to the 75W140? Add friction modifier to it?

Is it possible that the clutch pack discs in the rear cause a 4x4 shudder going straight on slick surfaces?

I've had the truck at the shop for a bit now (and payed a bunch of $ for exploritory diagnostics) and all they can tell me is 3 things.
1. We can't isolate the issue.
2. your front end components are good.
3. we want to just start replacing stuff and see if something eventually fixes it.

I pulled the truck from their shop and decided to do my own digging since they seem to have no idea what it is but want to charge me for it anyways.

Any help with this is appreciated. I'm typically a Chevy guy, but decided to try out Ford this time.

 

Go to the dealer and get a bottle of friction modifier put it in and drive it, its that simple. If your tire pressures are not even or you have different size tires you will have a shudder in 4

 

The correct weight oil is 75w-140 Synthetic. Add the Ford friction modifier. If the problem isn't corrected or stays the same you will need to change the clutch pack. You could change to the Eaton chunk which doesn't use clutches. It has gears that provide the limited slip that don't need the maintenance of the clutch pack.

 

If replacement is needed, the Detroit TruTrac (made by Eaton) is the best available limited slip option:

Performance Products | Detroit Truetrac Differentials

 

I had the same issue with my 2004.
I changed the, I am guessing, original fluid.
I used the royal purple 75W140 and added the ford limited slip additive.
The shudder is pretty much cured. Although now there is a noticeable whine at 50 mph from that area. Not sure if the two are related.
The shudder went away anyways. If the whine bothers me I turn the radio louder.

 

Burnout,

Had the same issue with my 2004 about a month after I bought it new.

Long story short is Ford had an issue with 2004 models not having the fric mod added at the factory.

Complained to the dealer-owner about being deceived the Service Writer and they replaced the clutch pack.