I don't know Silverados but I've seen the wiring harness question go both ways; some have manual trans and auto trans with different PCM's but same harness and they just pick up different pins to do the job, others use a different harness plus the PCM change. If I was in your predicament I would call a high level service tech at the dealer and ask; I've had some luck with those guys explaining stuff.

Sorry about your arthritis in the knee, but have you had it "officially" diagnosed, or tried any therapy? Although it could be temporary a cortisone shot can help. For my "minor" problems even Motrin helps. Many years ago I also recall thinking I was having arthritic pain in my knee and when I finally had it MRI'd it was a torn meniscus which I had cut out, in/out same day, cured it for about 15 years before arthritis started slowly setting in.