2004 GMC Sierra Rear end noise

Last week I bought a 2004 GMC Sierra 1500 from a private seller, seemed like a legit guy. Well he said that he had just had the whole rear end rebuilt, come to find out it was just the bearings and right axle. After driving it a few days whenever it would get warmed up there would be a terrible popping/ clunking coming from the rear differential. I spoke to the mechanic who rebuilt it and he said I would need to replace the clutch packs. Then when I was calling a few parts stores I had a guy tell me I might want to try changing the oil and adding the friction modifier since it was a limited slip and the mechanic probably didn't add it. This fixed the problem, but the past couple days I have noticed after driving it there will be a slight grinding coming from the back end when I turn the pick up off. Is this a big problem or something I don't need to worry about?