There are two speed sensors - they are basically magnets that "count" the number of RPMs before and after the TOD engages and feed back to the TOD ECU (TOD has a separate ECU that isn't read by typical OBDII readers). When these go out it definitely causes a check TOD light - it blinks and there are no indicators to the front, with the front-wheels disengaged. I went through replacing a set of bad sensors earlier this year.

If one or both sensors are bad, the TOD ECU should throw the check TOD light.
The ABS systems are connected to a separate ECU and do not use the same sensors (those are connected to each wheel and the ABS pump itself).

As suggested, the easiest "fix" is to reset by battery disconnect - this would at least clear everything as a cold reboot to the ECUs. Beyond that, I'm not sure how you can troubleshoot the issue. If you aren't throwing any check lights, the ECUs will probably not contain any data that can be read for diagnosis. Second issue, is that I don't believe the typical code reader can view or display TOD ECU codes (I had to go to the dealer to get the codes - cost me $90 which really sucked). I know for a fact that Autozone and Pep Boys both weren't able to pull the TOD codes, nor could my cheapy reader pull the TOD codes). Since all the electrical systems are double faulted to produce codes when there are problems (as in wiring shorts, ground faults or issues with sensors or ECU), I'm not sure where that leaves you. Hopefully the cold boot will clear things up.

-- John