Be sure your axle breather, located on top of the driver side of the axle, is clean. I had a plugged breather on my '51, causing a leak on the passenger side seal. I cleaned the breather, cleaned the gear oil off the drum and shoes, and four years later, no additional problems. As you drive, the differential heats up and the gear oil will be forced past your seals, if that breather is not open.
Others will tell you just replace the shoes and that may be best but I got away with cleaning with brake cleaner. (On the same truck, I had a front brake cylinder leak and cleaning the same way never got the brake fluid out of the shoes and I had to replace them).
