I'd definately be suspect of the rear main. Just fixed this problem on my dad's '01 Silverado, 5.3L. Keep in mind the "rear main seal" is not the back of the oil pan gasket, It's the actual seal around the crankshaft at the rear main bearing cap. It requires removal of the transmission, or the motor to replace. All the 4.8/5.3/6.0 motors from this era I know have a large problem with that seal. My '07 5.3L is starting to show signs as well
.