I do not know when it started leaking, but it was significant enough that it was dripping through the underbody shield. It had sprayed fluid over the transmission, bulkhead, and other components because it was leaking onto the axle. There is a Ford TSB on this issue. The real issue is axles surface finish that rides on the bushing. Due to poor finish, the bushing fails and takes the seal with it. It will get significantly worse with time. I do my own work, unfortunately this job is supposed to be done with special tools that cannot be rented. I was able to complete the job with tools that were similar and could be rented at your local auto parts store. I highly recommend replacing the axle as well as the seal and bushing, I purchased all of these at my local dealer for around $150. The tools I rented to complete the job were: axle puller/driver, 32 mm impact socket, torque wrench that goes to 250 lb-ft, bearing/seal driver set, slide hammer, pilot bearing puller slide hammer adapter. Only use MERCON LV fluid! This is a job I would typically recommend non-professions have done at a reputable repair shop.