Ever hear the old joke about the guy who decided to swim across the Pacific ocean. He got 3/4 the way across, got tired, so he swam back.
I would suggest that if you are going to go to the trouble of removing the driveshaft, the cotter pin, the nut and washer, you go the remainder of the way and remove the flange and replace the seal. Since you are talking about an output seal, once the flange is off, you should be able to pry the seal out with a seal puller or hook type slide hammer.
There is a seal puller pry bar that is relatively inexpensive, and achieves great success. An example would be the snap on YA-105, but there are cheaper versions of this.
http://www.ebay.com/itm/Blue-Point-S...item19cd12ee7c
On re-assembly, put the RTV (stands for room temperature vulcanizing, which is automotive silicone) around the shaft, let it "skin" for a bit, then install the washer and nut, effectively spreading the RTV to fill the cavity between the shaft, the flange and the washer.
In an ideal world you would use a flange holding tool so you could torque the nut on fully. But since most guys don't have justification to buy all the special use tools, you can likely get away with an impact gun or a power bar with the truck in gear and the park brake applied.