If a mechanic finds gear lube inside both rear brake drums, what is the most likely problem?

When gear lube is found inside both rear brake drums, it suggests that fluid typically meant for the differential or axle is somehow leaking into the braking system. This is most commonly due to a failure in the axle seals, which are designed to keep the gear lube contained within the axle housing. If these seals are compromised or damaged, the gear lube can escape and make its way through the axle shaft into the brake assembly, contaminating the brake shoes and potentially leading to brake failure.

In this scenario, a plugged axle would not typically cause gear lube to enter the brake drums. Plugged axles can lead to other issues, such as difficulties in the operation of the differential or rear end noise, but they do not directly result in seal failure. Faulty brake lines and a leaking master cylinder pertain to hydraulic brake fluid systems, not gear lube. Similarly, a worn-out wheel cylinder could potentially leak brake fluid, but it would not explain the presence of gear lube specifically unless there was an unusual circumstance involving both systems, which is quite rare. Therefore, the presence of gear lube in both rear brake drums most strongly indicates a leak due to a plugged axle seal malfunction.