I never bothered asking them this, but I assume when they do differential service, the add the GM additive right? Anyone work at a dealer that could comment on this. If I was patient, I'd wait and call them on Monday... As in another thread, I get a little clunk out of the rear shifting, especially from drive to reverse and I verified it was not the u-joints today. So the rear gears appear old or someone may have left out the additive.

If the additiive was left out, could I add it myself?

 

Sure, you can add it. However to know if that dealer put in the additive, you will have to ask them.

The only time you would hear a noise from the additive not being there is when you are turning.

 

Already checked and replaced that. After disconnecting the driveshaft I can tell that there is a little play in the differential which is what makes the noise when I go from drive to reverse. Probably something I'll just live with. Figured the fluid may have something to do with it.... Sounds like it doesn't from the other reply I got.