Well...abs is not my strongest area of expertice, but after reading your post, if it were mine, I might go ahead and replace the sensor in the transmission that measures the rear wheel speed.
(since you've already done the others, and a little more $$ won't hurt)
But...is the speed sensor at the speedo head used for abs??
Others that have had (unsolvable) abs problems have simply unplugged the abs computer, and gone on with life. But you are probably like me, and want things to work as they were designed to.
Have you followed all the trouble shooting steps in the TSRM??
http://www.cygnusx1.net/supra/librar.../br/BR_01.html