Same problem with my 1993 LS400 - Trac light stays on. Eventually "check engine light" came on. Took it to my local Lexus dealer. Said I needed new pre-catalytic oxygen sensors, suggested also replacing post-catalytic sensors. I also had a crack in my EGR exhaust pipe. He explained that with bad sensors and bad EGR, the Traction control system will not function properly. I replaced all four sensors and EGR pipe, no problem in two years now. Very hard to replace EGR pipe. Lexus wanted to pull the transmission to replace it but found a mechanic who wedged the transmission out of the way enough to replace the pipe. Dealer said I could drive with both problems just would not get optimal fuel economy. Hope this helps.
Other common problems LS400s have are with time and temperature display "bleaching" out and with speedometer instrument cluster malfunctioning. Had both problems resolved with repair services advertising on Ebay.