I had a problem with this (and many other things) when I got my car back from TitanMotorsports. I emailed them several times about the ABS light and they responded saying that it could be due to the HKS trackless kit that I have. This did not sound right to me because, the traction control and ABS are seperate systems so I started to hunt down for the problem myself. I found out that all four of the ABS wheel sensors were disconnected. I plugged them back in and the problem was solved.
So, I would check that first. They may have been disconnected to perform some work on the car and just never plugged back in.
A while ago I drove tested a 94TT (with purchasing it in mind) and both trac & ABS lites would come on. I checked with Toyota service and was told that even though these are separate systems, some failure conditions (which I can't remember) may cause both lites to come on.
BTW, I decided not to buy that 94TT as Toyota told me that it COULD end up costing me a couple grands. Just didn't want to take the chance.
In this case I believe that one may dig into comp error codes to decipher...