Curious which scan tool you're using? I've read that some are picky about ABS and other non-powertrain stuff.
I agree with this comment.
After a bit of exhaustive detective work in my service manuals the other night, the reason that the Tech 2 is capable of scanning all modules(besides being a dealer tool) is that it has two separate communication protocols built into it.
The first protocol is what is called UART. It is very simple form of communication because it is a DIRECT connection to a particular module.
Metaphorically, it's like someone whispering instructions into your ear and that person responding in the same fashion. The key take-away is that this communication is intended for a single recipient and not broadcast to a room full of people.
The second protocol is what GM calls Class 2 Serial Data, otherwise known as SAE J1850-10.4 VPW
This protocol is a form of network communication between modules. In 2008, GM should have switched all vehicles from J1850-10.4 to being CAN compliant.
Controller Area Networks have higher speed buses and greater bandwidth.
In 1996, J1850-10.4 was perfectly suitable because the only modules that needed to communicate with one another were the ECM and the TCM, and sometimes the BCM.
As the modules became more robust and more information sent over the network, it became necessary to link more modules together on the bus.
Again, metaphorically, its like a small group of people talking as group became a room full of people all talking at once.
So, bottom line, scan tools that are capable of both UART and J1850-10.4 protocols cost more because it requires two different types of hardware and their respective software counterparts to talk to these modules.
If you study the Pin-out chart on the picture above, Pin 2 is the only pin that is Class 2 Serial Data. The VCM talks to the ABS on that bus because Pin 2 is connected to both, however, direct communication from a scan tool with the ABS module is only done through Pin 12.
Pin 14 is direct UART communication with the radio, pin 13 is direct UART communication to the Electronic Transfer Case module(if equipped), pin 9 is direct UART communication to the DERM(Airbag module), and pin 8 is direct UART communication to the keyless entry module.
Obviously new cars all link these modules together on the CAN buses so scan tools for new cars only need CAN functionality. The Scan Tool just becomes another module on the network.
Disclaimer: I'm not an electronic engineer, but I do work on aircraft for a living. I'd like to hack my modules just for the fun of it but it's not likely to happen anytime soon.