OK - leaking 8 and 9 inch Ford 3rd members were, even new, not unusual. While I've never had a problem with one that I've put together, others will and do. Fact of life.

Now, as far as having the pinion lash reset after replacing a seal - not necessary. You already have the crush washer or stack of shims set and it has already become a solid one piece unit which will not change if you disassemble it to just replace a pinion seal. Also, if this guy does these on a regular basis, he probably uses a solid spacer and shims to set the preload.

Install a new seal, torque the nut to spec with a small amount of sealer on the threads. Obviously, check the yoke for burrs as well. If there is any failure it isn't from just replacing the seal - you don't really think dealerships or garages replaced anything but the seal if it leaked plus they did it on a lift with the differential in place (and probably seldom even used a torque wrench if a couple garages I worked PT at in the '60s and '70s was any indicator)