I just replaced mine a few weeks ago, not too bad. Since you have to pull the driveshaft I went ahead and changed out ujoints while it was out. There are some write-ups out there for this and some say to pull the housing that the seal is in and others just pull the driveshaft and replace it while still attached to the Jeep. I didn't pull the housing off...
The difficult part is getting that darn seal out. I bought a $7 seal puller and that helped but it still was a pita. Putting the new seal in can be frustrating. Once you get the seal find something that you can use to seat the new seal in there. I've heard that a piece of exhaust pipe works well. I didn't have anything the proper diameter so I struggled with it for a bit.
All told seal and ujoints took me the afternoon but I wasn't in a rush either.
Al