I have been messing w/ my ABS/TCS for months. It's still not working, so take what I say w/ a grain of salt. In order to do a complete diagnosis/system check, you need a Tech II and univseral pin-out adapter. But there are many circiuts that you can check if you can read a DMM, wiring diagram, and can follow instructions. If you don't already have GM service manual, I recommend either gpona.com (free)or alldatadiy.com (annual subscription)--you MUST create account for both sites and both sites have electrical diagrams and step by step diagnosis process.
Even w/o Tech II or universal pin-out box, you can still check many of the circuits for continuity, and verify that your instrument panel, DIC, & wheel speed sensors, are working properly.
A few things I have taken to note: first, about a million different things can cause this mess; the TCS is supposed to ground thru EBCM; if EBCM has a problem it can dissengage TCS, the EBCM communicates w/ PCM with serial data line (if scan tool isn't reading EBCM you may look at those wires to start).
Brandon was big help when I was trying to get my head around this. His advise to just send to modulemaster may have a lot of merrit--they are not that expensive, relatively speaking.