well i did have a mech have a look at it last night while i was having my truvk inpected. He just listened with one of thoes stethascope looking things. He said it is definetly coming from the rear but is not sure weather is is the ring, pinion, or a bearing. But now i have a a tranny problem which is a little higher on my list. So, in a way you were close
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2000 Silverado LS 4.8, K&N FIPK, Gibson Extreme Dual Exhaust, Line-X Bedliner, Black Speed Grille, Viper Alarm System, Custom Audio system.... (need to update my pic)