Yeah, I tried again a few times with and w/o Acq.
Couldn't reproduce the same behaviour when Acq G3 wasn't running: Instead, Limewire G4 as UltraPeer crashed after Acq started up (logged--130+threads; no Console), but no web or mail loss--I tried. Restarted Limewire while Acq still running, Limewire went into leaf mode and I haven't been able to make more problems since. Quit Acq on the G3, and tried twice more to get Limewire in Ultrapeer mode--it's behaving.

Looks like two clients trying to hold down quite a few connections is needed first for me. Explorer was running on both machines too. Oh well, another weekend not wasted.