I think the locking diff would be my first guess also, but it could also be a worn universal joint, and even though it sounds like it's coming from the rear check all of the u-joints....sound can travel the length of the truck through the drivetrain.
I think that Mobile 1 is really good stuff so I don't think it's failing unless the diff is under-filled. The easy thing to do is check the fluid level, then maybe try some additive in the rear diff. Also, if you have close to 50K on the current fluid I would change it.
Jeff
"But officer, I ain't Speedin', I'm just flyin' low"
2001 Silverado 3500 Dually LS Ext Cab, 6.6L Diesel, Banks Stinger Kit, 4" Exhaust, VR Straps, Bilstein shocks, 255/85/16 BFG KM2s
2004 Suburban 2500 LT 6.0L 4x4, Bilstein Shocks, Federico front/rear sway bars
1996 Caprice 5.7L LT1, B4U Suspension, KYB shocks & Hotchkis sway bars, BMR RCA - alas I miss ya.
2013 Caprice PPV 6.0L