Thanks Blue Bowtie.
I bought the original set of shop manuals for it, and just as you said there are exhaustive checklists for most any problem, most seeming to end up with replacing one module or another. The only issue is they expect you to have a GM scan tool with all of its bidirectional capabilities. That would be nice
Still, those manuals are a bargain for the information about every piece in the car.
I also agree with you that a lot of those codes are "cascading", where a problem in one module affects how the other one works etc.
I noticed a lot of common errors in the list of codes. Since the RIM was mentioned so often I went ahead and fired the parts cannon at it and bought a RIM and DIM module. I got them used, so even if they don't help, it wasn't a major investment.
They should be here this week, so I'll post my results when I get them installed.
Mike