First I must say I know my 05 Screw 4x4 had 208K on the clock and that the diff has wear on it. I'm the second owner and have the benefit of having the original owners extensive maintenance records. The truck was mostly a highway vehicle as the PO was in sales. I recently replaced the worn aggressive tread tires with all season radials due to my needs. Since then I have a very very noticeable whine from the diff on deceleration. My mechanic that specializes in diff's put in on the rack and isolated it to rear end. Then we went for a drive. He stated it's wear on the gears but not bad enough to completely rebuild at this time just annoying. I don't loke anoying especially on road trips. I was researching the maintenance records and even though the PO had the diff serviced at 40K intervals the last mechanic to service it with 34K currently on it used 75W-90 lube. I believe this probably is causing the largest percentage of the whine. I have seen the TSB stating the need to go to 75W-140 so since I had recently purchased LUCAS 75W-140 Synthetic lube and Motorcraft XL-3 friction modifier and will order a diff gasket and hope this does somewhat quiet my whine. I did notice that all service until the last one had used 75W-140 lube. I believe the whine was there all along but couldn't hear it due to the worn aggressive tread tires. It'll probably a couple of weeks before I get it done due to rebuilding the front suspension but will report back on my findings.
Thanks for listening this is a great forum.

 

Since he used the wrong fluid, it's very possible he forgot to add, or didn't add enough friction mod which could cause your whine. Let us know what happens when you get it done.

 

Pretty sure the 140 will mask that whine. Did he have oversized tires on it or just mud terrains in the stock size? Reason I ask is bc if they were too big and he didn't re gear the rear end, he may have done some damage. Check the gears when you change the fluid and look for obvious wear. Or could be as simple as SCORGE said, the wrong fluid was used with no friction modifier.

 

I had a whine on my rear diff after loaning it out for a long road trip. I thought I had heard a little of it before they borrowed it, but it was very apparent once they got back from a 2000 mile trip. I changed oil, checked gears, all gears felt like new, so took it to a mechanic who found the rear pinion race had holes in it, and even the pinion was difficult to turn without the carrier installed. The holes were likely caused from heat buildup. The mechanic replaced the pinion bearings, new oil, seals, etc, and its working perfectly so far. Mine was the 9.75 rear axle.

 

Do you have a LS rear end? Do you have any low grumbling when turning a tight corner very slowly? These LS rear ends are known for having slight clutch pack problems. My wife's Explorer has the LS and when you turn a tight corner slowly it has a light grumble. It also has a light whine in the rear when decelerating. I removed a little of the oil in the rear and added 4oz of MC friction modifier. The grumble has almost gone away and the whine has gotten much quieter.

 

Yes I have a LS diff but there's no grumbling or ratcheting when turning the corner. The fluid is clean and clear it's just too light weight of gear lube.

 

Well I replaced the diff fluid but it only helped a little. Sometime in the near future I'll probably have it rebuilt.

 

Most likely the clutches are gone in this case.

 

Any idea what it would cost to replace the clutches and seals?

 

Sounds like time for a trutrac upgrade