The key to successfully replacing the hub seal (assuming it is the inner) is to install it in the hub very carefully so it is straight and so the inner portion of the seal rotates freely with respect to the outer portion.
If possible use a hub seal insert tool, but if none is available just be certain when it is tapped in that it goes in straight. I use a 6 X 6 block of wood and tap it in by going all around the perimeter slowly not allowing it to get cocked. When you are done it is vital that you insure the hub has oil in it and if you want to be certain you will not run the bearings dry grease the bearings so they are pre-lubed.
It typically takes an hour and a half to two hours and it is not particularly difficult. Any truck mechanic or a mechanic that works for a charter bus or school bus garage can do the job.
Did I say to make sure the bearings have grease either by filling the hub or greasing them? It takes forever to hub oil to oooze past the bearings so be patient when filling the hubs with oil.