I replaced the input shaft seal on mine just last week. It was leaking badly, now no leak at all. You do not have to disassemble the entire gear box to replace the input shaft seal. You do have to remove the input shaft though. You could probably do it on the truck but IMHO it will be much easier with the gear off the truck. I followed instructions from here just to get the input shaft out.
http://www.scribd.com/doc/6829240/Sa...Rebuild-How-To