The car was just in the shop for an alignment and this problem was first noticed by me when car picked up. They said the rear sway bar (after market) had broken when on the lift. ???
While accelerating straight the anti-skid light comes on and goes off when brake on 2002 Lexus SC430
check the speed sensors could be getting different signals from them or bad connection could have got hit when doing alignment
A scan tool (even generic) with Lexus-Toyota capabilities may be needed to interrogate the computer in your car to see if the ABS sensors are at fault or if the steering yaw rate sensor needs to be calibrated after the alignment.