So I took my bike down to the LBS. The guy wanted to check everything out before selling me the spacer to make sure I wasn't doing anything stupid. He tightened down the cog w/ a chainwhip and then the lockring, he had the hozan tool.
Sure enough, the cog could be loosened a bit with the lockring tight. He seemed to think that the original cog was on tight enough to compress the aluminum that it tightens against, making extra room. He put on a thin spacer, and tightened up everything real well. The cog could not be loosened with the lockring tight. He told me to take it home and sprint a bit, coast to a stop, then tighten the lockring some more (I have the park tool).
I did just that, then took it out to try to skid. First try the lockring popped off, stripping that section of the hub. The cog gently unscrewed.
Thinking back, I (maybe the LBS guy too) probably tightened the lockring way too tight. When I was tightening it, it was on very tight, but I could make it budge a little more, it kept budging a little more with a few more pushes, but didn't feel like it was getting tighter. I only budged it 3 or 4 times. Also, with the spacer, there were less threads for the lockring to engage on.
I guess I'm SOL. Anyone buy wheels from www.bicyclewheels.com?