I had the fluid changed in my 9.75" rear differential about 1000 miles ago and since then it seems to be dragging around tight corners. Some corners worse than others and sometimes its really noticeable.

I had the fluid replaced with Royal Purple synthetic with friction modifier already in it. The correct amount was put in.

I added a tube of friction modifier about 500 miles ago to see if that would help the issue but it didnt.

What could be the issue and what needs to be done to fix it?

Are the clutches shot? The truck has 121k and the rearend was fine until getting the fluid changed as preventative maintenance at 120k. The bearings were all replaced at 96k due to wear.

There also seems to be a slight vibration on highway speeds but Im not sure if its the differential or a slight skip in the engine (truck seems to run fine though). I scanned the truck for codes and nothing came up so Im assuming that its not an engine issue.
Would a clutch pack issue cause this issue? It seems to happen at/around 45 mph and up. Nothing too serious, no shaking in the steering wheel and no issues while braking (rear rotors arent that old).

 

no ideas? Its rather annoying how it drags.

 

Is it particularly bad when turning corners at low speeds?

 

yes, thats when its at its worst. Especially from a stop like at a light.