Nice work, glad to hear you may have found the issue! Yes, an overfilled differential can leak through the pinion seal. A pinion seal, like any other that seals a rotational shaft (front crank, rear main, axle shaft, etc.), is dynamic. Meaning it is not designed to be stationary with fluid sitting behind it; the fluid will eventually pass between the seal lip and the component. Overfilling can also cause fluid to bypass the seal, it's the same principle that causes a rear main seal to leak when a PCV system is inoperable; the pressure will find the weakest link. Let us know if the issue is resolved once you get a chance to re-evaluate!
