Uneven rear shoe wear is actually pretty normal, to a reasonable level. Thats because the action of the brakes when contacting the rotating drum causes the spring system to push out the lower part of the front shoe to make contact. As the back shoe is being pushed from the top, the ability of the lower part of that shoe to connect with the drum is diminished. Good brake springs, including the equalizer bar spring, is supposed to make up for this natural discrepancy, but it doea not. Soooooooo, long winded as it is, if the back shoe is greatly different from the front shoe, change all your springs. Hold down springs, when weak, cause side to side wear thickness. Bottom , top and equalizer bar springs are for front / back wear.