Answer:
At NAPA you can buy split backer plates for around $50.00, you do not have to remove the axel. Four bolts and release the old backer, slip the new two piece backer in place and your in business.
Yes, you can replace just the backing plate, they are available from rock auto for about $11 each. The bigger question is what is involved in changing out the backing plate, a repair job that I am faced with as well. I have yet to perform the repair - and cannot find any information online about it either. At first glance of the truck, my best guess is that the axel shafts have to be removed, and then 4 bolts at the end of the carrier tube that hold both the caliper bracket and backing plate. I am hopeful that I do not have to disconnect the brake line or emergency brake cable to swap out the backing plate. While I still think my Silverado is a great truck, I think that Chevy should have used a heavier piece of metal, especially given the fact that this seems to be a common problem with these trucks...
Changed the backing plates on my 03 Avalanche yesterday.
Drain differential, remove the small bolt (8mm head) in the center section, and push the round pin out far enough so the axles can be pushed in towards the center. This will enable you to remove the u clip that holds the axles in place. Rotate the center assembly a few times and you can see how it is done. Biggest problem is the parking brake cables. You will need a tool to compress the spring so the two small tabs can be depressed on the plastic end and pushed through the bracket. Do not start the job until you can easily depress the spring. Use a hammer and chissel to crack the rust layers form the inside edge of the rotor, otherwise the rotor may rub on the new backing plate. The e-brake shoe can be removed with the axle in place, but you have to rotate one end off of the axle. The small adjusting lockplate behind the adjustment wheel is a pain to release. The e-brake adjuster arm is connected to the adjuster buy a push pin. Do not lose the pin. Screw the adjusters all the way in because the rotor inside e-brake surface is generally rusty. Also replace the axle seals while you are at it. When the u clips have been put back onto the axle ends, and the center pin locked back into position, place rotor on axle and rotate to ensure the rotor is not rubbing on the shoe or the backing plate. Check rear pads to determine if excessive wear on one side only. This indicates a binding caliper.