Before you start in on that rear crank seal, better read up on the procedures. It will require you to split the engine case and pull the crankshaft. When you do that you should replace the stretch bolts on that assembly.
Will take about 12 hrs with the engine out, and those bolts will cost a hundred bucks. The rear seal costs only about fifteen. It is held in place with pins on both the top and bottom of the engine case, so you can't just slide them in and out.
If it needs to be done, do it now. But that is a big job to do just to make sure of things.