No problem at all, that is if you have a few special tools.
First back off slack adjuster.
Remove axle nuts, smack axle with sledge, remove the taper wedge washers. Remove drive axle, have the coffee can and rag handy to catch oil. Jack this side of axle up to just clear tires from floor some, now loosen the bearing nuts some, this takes a big special socket. Either use a wheel dolly or oil the floor, if using floor just touch tire to floor and finish removing nuts. Rassle it a bit and the outer bearing will come loose, more oil will escape.
Slide the wheel assembly off axle. Get another coffee can and a big rag, put rag in drum and can on top under inner seal. Take a bar and pop seal out from other side by driving on inner bearing or use a seal removal tool and sometimes a rolling head bar works.
Stemco seals always worked the best for us in the truck shop where I worked, they have an inside wear ring, seal itself is made of rawhide. They take a special driver. CR's or Nationals are OK but we had much better luck with the Stemco's.
Do your cleaning up, steam cleaner time, replace shoes if too oil soaked. Might as well do the other side as well then.
Install wear ring on axle, place bearing in hub and install new seal. Slide wheel assembly back on axle taking care not to damage seal. Fill cavity with oil, slip in outer bearing, snug down nut decent. Jack up axle to clear tires from ground, then install the washer making corrections to get the holes to allign with the protruding pin on this inner nut. You want some preload on these bearings, not much but somewhat snug with this wrench, and the handle isn't that long. Wheel assembly should turn with ease. Snug the outer locking nut real good.
Put axle back in, a thin film of Blue Goo works fine. Make sure you get the tapered keepers in, add nuts and impact them down good.
Readjust brakes and you are ready to go.
About a 45 minute job plus cleanup time.
You probably have outboard drums and pulling the complete wheel assembly isn't necessary. You can remove wheels, then drum, then the axle and hub assembly. By removing as a complete unit it is easier to keep things straight and not damage seal when reinstalling hub. It seems easier to me.
Easiest way would be to take to truck shop, I've had to clean up these messes before, only thing that might be messier is a front pinion seal, either way it slings a lot of oil. 90w doesn't like to wash off trailers and backs of cabs very well. Plus we used red synthetic, might have been worse.
Been going on 10 years since I had to do this job, typing out the procedure might keep me in practice? Kinda enjoyed truck mechanicing in a way, sure miss those service calls in the winter, ya sure.