That seal can be finger pressed in. just raise the front end high as you can safely ( this will limit spillage ) , disconnect coupler, remove input shaft to chassis tunnel boot and crossmember if needed, support the gear box with a jack or stand. Use a small screw driver rapped with tape and work the old seal out.
Use silicon or assembly lube and slip it over the shaft and down to the nose, finger fit flush. put everything back. Nothing will go out of adjustment. That's it.
Leak will just get worse and you will have boot full of gear box oil one day if not addressed sooner.
While you are there, check level of gear box fluid but on level ground of course. Heck if she has not been changed, take the opportunity to change it, she will be happier
My 2 cents.