I have a 2006 supercab 4x4 with 111000 miles that I bought used, I changed the rear diff lube with the full synthetic proper viscosity oil, but skipped the friction modifier since oil claimed it unnecessary. When I turn sharply accelerating from a dead stop, I get a shudder, or tirescrub type vibration. I verified that the iwe on both sides is not engaged( not stuck in four wheel drive), tire pressures are all good too. Alignment was done, and problem is still present. Could it be the rear axle binding due to no friction modifier added? I noticed that raising rear tires off pavement, when rotating wheel on one side, the opposite side should rotate opposite when truck is in park but doesnt. When in neutral, both tires rotate same direction, much effort to turn one wheel in both instances, and the brakes are not dragging, it is the same with the rotors and calipers removed. Truck rolls fine in neutral, and can be pushed relatively easily, so no drive shaft or u joint woes. Any guesses?

 

If both rear tires rotate in the same direction when you turn them in neutral, you have a Limited Slip Rear Axle. You must add the friction modifier or you will get the shudder you're experiencing.

 

Have your axles checked.

 

You need to add the friction modifier. Go to the Ford dealer and buy a bottle, remove the top rear axle plug and pour it in. Then drive some figure 8s.

 

http://www.ebay.com/itm/like/310303114133?ul_noapp=true&chn=ps&lpid=82

 

Thanks for the help guys, figured it was the additive, I'll get it added.

 

Just joined the forum, I have a 2002 XLT 7700 Series with 93000 miles I bought last September. Today the rear end started shuddering when turning from a stop. Do I need to change my rea rend fluid or just add a bottle of friction modifier?

 

With 93k, assume it's never been changed - I'd change it and add modifier. 75W140 synthetic (any brand) and Ford modifier. I believe the 7700 has a 10.25" rear and you may need 2 bottles of modifier.

 

Additive is in, noticed diff runs a bit quieter, still giving a bit of shudder on those hard turns from dead stop, I'll keep watching for more info on this. By the way, I checked at a local parts store for the friction modifier, they had a brand X for 10 bucks, so I went with glc's recommendation on the Motorcraft additive, 6.99 out the door at the dealer, feel better about it too. Only 4800 miles on the 75-140 Valvoline full synthetic that I put in back in Sept. when I bought it, and it came out looking great still, good color, 4 oz. out to put 4 oz. of additive in.