2001 F-150 4x4 4.6L Off Road 19 for rear differential. ~68K miles limited slip front differential.
Not a single word in owner's manuals about servicing until 150K miles or ever adding additive. Lube for front is not synthetic.
And over the last couple of weeks I've noticed the jumpy walk when turning which is typical when needing the additive.
Does anyone have updates on this problem or situation?

 

Due to the age, it certainly would not hurt to drain and fill the lube on the diff. Be sure to use the Motorcraft XL-3 friction modifier since you have a LS. Drive it around in a couple circles in both directions and report back.

 

Front differentials do not have limited slip. 19 means rear is not limited slip. If you are not the original owner, are you sure that the rear was not converted to limited slip?

The BEST lube for the front is 75w90 synthetic. Do not put additive in it. Rear should be 75w140 synthetic, no additive if it's not limited slip.

To determine for sure the rear is not limited slip, release the parking brake, jack up one rear wheel and put it in neutral, try to spin the wheel that's jacked up. The wheel and the driveshaft should turn freely. If there's a lot of resistance, then it is in fact limited slip.

 

My Dad bought new. Definitely not limited slip on rear as indicated by axle code on door tag as well. I've driven plenty enough in 4 WD to know front is limited slip. And now that when turning the clutches try to lock indicating the additive is worn out.

 

No, the front is *NOT* limited slip. Ford did not offer limited slip in any front differentials in that generation truck. I'd check your CV joints.

You WILL get chattering if you try to drive in 4wd in anything except slippery conditions.

 

Yes there is excess play on inner joint of right side and on the a-roun-to-it list.
Not limited slip?
Hopefully you know the functioning in 4 WD better than I do then. So, just how just the front axle behave when 4 WD is engaged. How is it that both wheels in front will pull no matter how slippery the surface? That is, does the front differential behave like a normal differential in 2 WD except the input from the transfer is not engaged. Just spinning away with the front wheels allowing for differences when turning.
But when 4 WD is engaged, input from transfer case is added and the differential becomes locked?
That would be only other logical explanation for behavior I have seen.
And if so, I've got some troubleshooting to do. Because in turns it acts like a locked differential or limited slip clutches not slipping.

 

Believe me, the front diff is a standard open diff. This video may help:

 

Thank you very much. I have watched several videos but this one seems to be the clearest and with good details of steps.
Absolutely correct that I don't have limited slip. I just did not think that using a locking differential in a vehicle capable of high speeds would ever be done. Danger possibilities multiplied when you add speed. But unlike my Dad I probably won't be attempting 40+ MPH on snow and ice covered roads.

Now the ugly side of current situation. Whatever was causing it to engage on it's own has corrected itself. The most difficult type of problem to fix. A visual inspection and reading of relay values is about all I can do until it acts up again.
At least I'm now well prepared for drain and fill.
Thanks everyone.