Final Update:
Pumped in 3 quarts of gear oil through the VSS hole today (well I held the hose and the bottle upside-down and made my daughter do the pumping). Went around the block and got a howl when decelerating.
I did some research and found this meant the pinion nut was too loose. I thought I'd over-tightened it 1/4 turn, but apparently it was 3/4 of a turn too loose. Hard to judge, due to the press-fit of the flange. The site I found this info on listed symptoms for all sorts of diff noises. Pretty handy info. Apparently it takes around 200 ft lbs to compress the crush sleeve, so anything under say 150 ft lbs on the nut and you should be okay. It's almost impossible to get anything near 150 ft lbs lying on you back in the driveway so I just tightened the pinion nut as much as I could. There was a point at which I couldn't budge the breaker bar any further so I guess that's how tight it needed to be. Took it for a spin around the block and no more howling noise. No leaks either, so I'm a happy camper. I Learn something new every day. What did we ever do before the internet?