It is not all that hard if you are mechanically inclined and have tools, but unfortunately it is kind of pricey. Mark In AZ (moderator of this forum) has done it. You can get an idea of cost from these links...

Mark's article

http://www.offroadsolutions.com/technicalarticles/manual_hub_conversion.htm

http://www.parksoffroad.com/tacomamods/orsmanhub/orsmanhub.htm

http://www.sonoransteel.com/junk_yard_manual_hubs.html