A few weeks ago I was alarmed by a one-per-rev banging coming from my left rear wheel while I was driving up a tight-radius parking garage ramp (and posted a message here to that effect). I finally got around to pulling the rotor, expecting to see a greasy mess from a failed bearing or seal, or possibly a broken spring on the parking brake shoes. Nada. While I had the back end off the ground, I checked the diff oil level - just fine. But while rotating the back rotors, I realized both were going in the same direction. Hmmmm... then I pulled the sheet metal tag off the diff, soaked it in solvent to get past the globbed-on undercoating. Lo and behold, I have a "no spin" differential! Even though I have the original window sticker from the PO, there was nothing on there to indicate that the optional limited slip was installed, and the PO never mentioned it. Last summer after I bought the car I treated it to fresh fluids all around, including Mobil 1 gear lube in the diff which is what's in there now. I went thru the archives here and found nothing to suggest that synthetic gear lube is wrong for a limited slip unit, but the loud stick/slip noise I think is telling me something. One contributor here said his diff only quieted down when he used the pricey stuff Volvo sells. However since Volvo ain't in the lubricant business I suspect there's something else available on the market. Any suggestions?