It started about a year ago. Making a tight turn at low speed it felt and sounded like differential was sticking as the short radius wheel was not disengaging. More of a feeling than noise. I call it a chatter feeling like a partial disengagement. I recall the LS was not working as well. 2003 Ford F150 FX 4x4. Ford dealer wanted to replace discs I recall about $500-$800 is my guess on the estimate. Local mechanic suggested adding an additive and that I believe lubricated the discs, he did, and all was well. This winter(WI) it appeared my LS was not functional again. On a level street with ice under snow drivers side wheel just spins. I believe it should throw traction to pass side but does not. 4x4 allowed me to pull off. Are clutch discs just worn out?

What should I expect on repair cost-parts? Should I use Ford parts or aftermarket?

 

The additive is like any other additive, it looses it's use value as it ages and is used. Drain the rear diff again and add the additive. You can even add a bit extra additive (not 2 tubes or bottles) but 1 1/2, and it will sometimes give enough additional adhesion (best term I can think of) and cause your plates to engage more readily. At the most you'd be out 20-25 for the fluids. I'd rather spend 25 now, then 500 or more later. I have a F150 that also made noise from the diff at 25k miles. Refilled and added the additive, worked fine until 100k. Redid it and it's still working fine today. That was 6 years ago and it still works great. Original Ford differential plates are some tough parts and rarely have to be replaced for wear. The diff's will blowup (as will every other manufacturer), but the plates rarely need replacement themselves.

 

This past weekend I upgraded my diff clutches to the cobra carbon fiber clutches. $100 from summit for the kit, and about $25 for fluids and rtv. Took about 3 hours.