Been there done that! Both the front & rear seal on my T-case leaked like that.
The seals are easy to get at Napa/Kragen whatever. I have a NP241 but I think yours is different or I'd get you the part #. Just look for rear output shaft seal for the T-case.
T-case outputshaft seal, $11.99 which seems right!
You're going to want to get a seal remover tool. Look in the parts section. It's like a big claw hook thing. Works well for that rear seal. You pry that one out and then you get a big socket or a seal install tool and hammer it home and make sure it is seated flush like the current one. You WILL loose some fluid doing this, so take note how much drains out OR jack the rear end up but still you're going to loose some. Buy a quart or 2 of ATF to refill it and bam done.
Also wipe off anything on the driveshaft slip yoke. Make sure it's smooth so it doesn't have a rust spot or something to tear the new seal.
Everything always takes me a long time. I think I remember it taking me an hour? or so but that's the first time I attempted that. I imagine now I could have one changed within a half hour with clean up. Mine went out at 100k miles, some seem to go though them alot. But atleast they are easy to change.
Good luck, I'll help where I can!