I think you were dealing with a low factory fill. GM is notorious for under filling the diffs. Mine was 2 quarts low when I bought mine back in '01 and my brother's '07 was the same way.
Mobile 1 synthetic gear lube is top shelf stuff, and 75w-90 works well unless you're really towing/hauling heavy. Your camper/Wrangler combo probably weighs in the 7500lb range, so the 75w-90 is adequate in my opinion. I'm not a Mobile 1 tech, but I don't buy the 75w-140 blowing out seals.
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