05 GMC drum to disc swap?

[Flying Hamster]

Either there is something wrong with your rear axle (bent shafts, flanges come to mind) or you drive an awful lot of miles in a year and a half. I'm at about 120,000 miles with the original rear brakes.
FYI, the factory rear disc brakes are notorious for premature wear, dragging, E-brake issues, etc. This is why the factory went backward and returned to drums. Warranty nightmare, at least in snowy areas.