Swapping parts to test if something is broken is not what I like to do either. However, perhaps you can get a TCCM from a wrecked Blazer as an inexpensive part to swap?

 

I had this problem, all 4 lights on. The TCCM was baked. I changed it from my parts car, and now it works normally.

I'm posting this cuz I saw this problem mentioned a lot here, but never the solution.

 

If your scan tool can't connect (and it's good), neither can anything else in the system. At least take a peek at the TCCM to visually check for fried components, it might help narrow down the search for what's wrong IF it burns up a junkyard TCCM as well. It's always worth checking your local pick n pull for spares and backup parts, sometimes you find a minty low mile one that got side swiped and totaled out. That's how I picked up a perfect driver's side door, door card (with non-broken pull handle), window, motor, regulator, switches, everything... for under $50. The 4 button 4x4 switch, digital HVAC controls, TCCM. Heck I even grabbed the unused jack and tool set out of the back! In all honesty, if it had been hit a little farther back, I would have pulled the engine and transmission out of it but the engine bay was 'a little more cramped than usual' from the accident. Don't be afraid of 'junkyard parts', just figure out what put the vehicle there in the first place (aka make friends/be polite with workers there) and then pick parts around it.