Had my grandfather ride along in the backseat last nite to confirm that the noise, was indeed the differential. He said that that diagnosis sounded exactly right. drained the diff this morning of the week old mobil 1 75w90....the fluid was clear in sample, but black in the waste container. flushed it out with a bottle or two of 80w90 cheapo stuff. drained and waited...put in 1 3/4 bottle of royal purple 75w140, and a tube of diff stuff from autozone. drove it, found that it was making slightly more noise now. got pissed off at it. drove it home, and on a lark, put the two out of round tires on the front of the car. test drove, with cautious optimism. the "diff" growl had shifted to the front of the car, at least to my ears. swiped grandfather for test drive, to confirm. told me that the only sound he could hear from the rear end was the sound of my flowmaster/holey OEM muffer leak i doubt, in retrospect, that i needed to do much more to the mobil one than add a tube of LSD additive to it for it to work well. however, as i notice no measured loss of fuel economy or acceleration, will repeat in 30k miles with the 75w140. will be replacing the two out of round tires and getting a bent wheel straightened in about 2 weeks, hopefully that will kill the sound.