(meant to post this last night... I see others have mentioned some of these things now, but still...)
The gauges are controlled by things called stepper motors. Check out some of the You Tube vids on doing those. They're a bit tedious to do, from what I hear, but some people here have done them successfully.
As you've seen, there's a nice little cottage industry in rebuilding GM truck clusters. Between the cold solder joints, the stepper motors, and even the lamps, people have probably made enough money off fixing GMT800 clusters to put a kid through college.
You may even find a local Craigslist source, if you're interested in that. There are people in my area who do them, swap out the lamps to LEDs, etc. On my Sierra, the PRNDL display is out (at least, in daylight; I can see the ghost vestige of it at night, if I turn up the illumination all the way. But everything else works, so I haven't gotten it fixed, yet (I'm doing other projects on it, first.)
The '03 Silvy cluster should swap right out, especially if the trucks are optioned the same (steering wheel controls, no trans temp gauge at the bottom LH side). The odometer should display the Chevy's mileage, not the Sierra's. If one of the trucks doesn't have the SWC, then I don't think it will have the Driver Info Center (DIC) messages displayed - and I wouldn't leave it connected very long.
GM re-engineered the cluster in '03, and it remained the same until the GMT900s were released in 2007.