I discovered that the K2500’s timing wire has been disconnected the entire time we’ve had it. Hooking it back up fixed the idle issue and has made it drive like a new truck. It’s got lots more power, and the fuel economy is much improved (the last fillup was 12.9 MPG, up from a low of 6.6 MPG and from a previous fillup of 10.0 MPG).
And for the first time, the “service engine soon” light has turned off.
I still haven’t installed the new fuel injectors or O2 sensor.