well I see that many people suffer from this "shudder" issue and I developed the same problem a short time ago. It does not seem like a uniform issue though, I read many other folks with this issue but a different problem and different symptoms.

In my case, from a stop, when I accelerate only in a turn left or right I get a shudder and shake from my rear end and it seems to be worse when the truck or air temp is colder.

well I went with the most basic thing first, popped the diff cover off, drained all the fluid, wiped out as much as I could get out of the collection area in the bottom. put a new gasket on and a little rtv, replaced cover. pulled the plug and cleaned off the shavings from the magnet and put new oil in it (I used Valvoline 75-90 with the friction modifier already in it, said limited slip right on the bottle)

test drove it around the block about a hundred times making left and right turns and the shudder and shake seemed to constantly decrease untill it was perfectly smooth and noiseless!!! so that seemed to be an easy fix however I will report back to see what happens when I take it out cold in the morning

 

oh btw I have an 04' 150 4wd with 90k on it with the 5.4 motor

 

Well, that is the fix for a limited slip differential. I would have used Ford fluid and additive if so specified, since you'll probably not have to change it again. How about that front diff - ever changed that one?

 

My truck does this vibration from a dead stop. And does it a little more noticeably when turning sharp left and right from a dead stop.I will try this fix but I have one question. Did your rear diff ever make a faint howling noise at certain speeds?

 

Differentials can make a light howl at certain speeds without there being a serious problem, but the fact is that it is either worn or not set-up exactly right. I might go with a heavier oil in a noisy one, but don't go overboard with that or the oil will be too thick in cold weather. Maybe a multi-vis like 75-140. The other thing is, determine at what speed/conditions you are hearing the howl - drive, coast or float?

 

My howl is happening when accelerating between 40-60 mph. Doesnt matter if I have it to the floor or feather footing the accelerator. Howl is faint though. It doesn't change when RPMs change.

 

well my problem has improved but not gone away. still shudders a bit when its cold. thinking I should maybe add a bit of the friction modifier? or maybe change the fluid in the front diff? havent touched that one yet

not sure about the howling, I dont have that with mine, how many miles do you have?

 

Most likely the front end is not limited slip. I would see if Ford has an additive for it.

 

That's exactly what my old Cherokee did - I would get that light howl as I entered the interstate and accelerated through 60 MPH - after that it was not apparent. I never did do anything about it and it never changed.

 

I also had the "shudder" when I purchased the pickup two years ago. I changed to royal purple 75-140 and added the ford limited slip additive. Problem gone. I also have the "howl" issue at around 50 MPH upon acceleration. This is going on about two years and 20,000 miles, so apparently not serious enough to stop the machine. Anyone fix this issue? How did you fix it?