My approach would be more direct;
Mount motor, if no shaking
Mount pulley to motor, if no shaking
Mount belt to next pulley/idler, if no shaking
etc.
It could e.g. be a pulley with out-of-centre hole thus acting as an out of balance flywheel. Or a really badly made v-belt, with lumps.
You have already eliminated the motor itself from your previous investigations.
Given the simplicity of the drive train it doesn't seem worthwhile doing the 'binary search' approach that otherwise works so well.
gerrit