I don't know where its coming from forsure if it drive shaft or clutch ?I know when you on gas hard through the gears you can feel it. then when say going 55 to 60 and you push the clutch peddle in it feels like it gos away.It has a set of richmon 4.10 gears in rear I did have to replaced bearings soon after put them in because it was rummbling so the gm garage redid them?