Well, my guy traced the problem. I guess this would benefit EVERYONE who changes the sway bar and/or bushings on their 1995-2000 LS400's due to a squeaking sound. The problem is nothing to do with the actual sway bar bushing, its the bolt that goes through the bushing. Mine was rusted, so that causes the squeak. Took some sandpaper and cleaned the rust off, spread some grease over the bolt and slipped it right back in. Viola! Instantly no more squeaking!
I'm surprised no one ever figured this out. Mechanics will all tell you replace this and that part. If I had known, I might not have even wasted the 200 on sway bars with bushings already pressed in and the money on labor to install it. But my car swayed a ton in the wind and the bushings were cracked so I guess there was some benefit from it.
Otherwise, just getting the bolt out isn't a huge job, clean the rust off of it, slide some grease on it and push it back in and tighten it up. No more squeaking!
Or you can just buy a new bolt from Lexus. Hope that saves someone else some trouble.