In regards to those shims (IF that's the problem) the newer type, current rear brake pads (with rubbery backing) supercede the older pads which needed those shims so if one gets the latest pads, one doesn't need those older type shims (old TSB info from '98 I believe).

Another possibilty is that the car might have a sticking front caliper piston which only sticks (siezes) when the brakes get hot, usually only one side and discovered by trying to turn the wheels while they're off the ground. The sticking one also usually discolors that rotor and the wheel area get VERY hot.