Stabilitrak was introduced on the Trailblazers in 2006, and a very common cause of the Stabilitrak light is a faulty steering wheel angle sensor. Might be something to check. I have no idea if his van has one, but if so it could be the issue.
It does have a SWPS it's now showing code C0455 after changing the pitman arm.
Van had 47 codes showing the first time I hooked the code reader to it accross ECM, ABS, & SRS modules. With that many codes showing I decided to clear them all, and see which ones returned. The code for the passenger side front WSS was the only one to return along with the SRS codes. So we replaced the WSS and that cleared up most of the "Limp Mode" type symptoms (and the code). It was acting like it had a bad TPS with a bad stumble off idle when you would give it gas to go, along with what seemed like a miss/lack of power around 45-50 MPH. Ordered both another front WSS, and TPS. Replaced the drivers side front WSS (easiest of the two to replace) and the stumble/miss lack of power problems were gone. (Didn't get a code for either)
After going through similar problems with our 06 Honda Pilot (Bought from neighbor I'm helping) and buying a code reader that was "Supposed" to perform programming/maintenance functions but couldn't,
I was basically given an older laptop system by a mechanic friend of mine that the laptop had been dropped/broken. Asked around and was given an old Dell D430 laptop that I changed over to a SSD and loaded W7Pro and the HDS software on. I'm still trying to figure out what auto brands some of the software, and interface modules are for. I managed to ID the Honda stuff. (Everything was stuffed in a box) He bought it as a system sometime around 2010 from a guy that was building/selling them. He told me if I can get it working to keep it, he now uses one of the subscription systems. IIRC it's an All-Data system.
Neighbor called a GM dealership, and was told $180.00 first hour, $160.00 per hour after that for diagnostics/recalibration of the systems. I told him about the Tech2 diagnostic system, and since he has 5 Chevrolet products that fit the systems time frame (92-05) and after seeing what the HDS/HIM system is capable of compared to the scanners he asked me "Can this "Unlock" the radio in the 05?" After telling him yes, he decided he needs one.
Now I need to find a pdf of the FSM for the 05 Express so I can bring it up on my secondary 12/110 volt monitor so I can read the required procedures to do what needs to be done.