It's too tight in there to do it on the truck...I would drop the PTO off and do it right. It's not that big of a deal. You will need to drain the transmission, so this is a good time to change the lube in the tranny.
You might as well also replace the rest of the seals on the PTO while you have it out...OD Iron sells a kit with the seals and boots. You can do this with hand tools (you will need a small-ish gear puller to get the output fitting off the shaft to get the cover off front of the PTO and get to the output shaft seal.
Note that you will need a new gasket when you drop off the PTO. You can get it from NAPA if you ask for either Victor H26205 or FelPro FPG 55379.
If you have it all lined up I bet you can get the PTO off and gone through, and re-installed in 4 hours if you have all the right tools and parts ready to go.