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Everything above is really good advise. But unless I missed it no one really touched on a couple different things.
A properly lubricated seal does not go bad for no reason, worn out bushings and or bearings cause them to fail. Be sure to check for excessive slop in the output shaft, very common for rear case bushings to fail and cause the seal to fail.
Also on the weep hole leaks in the pinion shafts, If you develop a leak in this area I highly suggest you check for a plugged transmission case vent tube. If you have a plugged up vent tube and you plug your weep hole in the pinion you can have several future problems arise
High case pressure causing fluid to blow the dipstick out of the tube and loose fluid there, older weaker valve body gaskets can blow out, as well as input shaft seals.