i had the transfer case shifter freeze up on me a couple of years ago (96 disco sd 5speed). the truck used to be my father's daily driver, and he didn't exercise the t-case enough.
basically, the handle is connected to a splined wheel with a collar that rotates forward and backward to actuate the high/low range. there is a rod going through this wheel that moves side to side to engage diff lock. if its still moving, you should be able to lubricate it without taking the whole box out. the site andy gives is a very good reference.
personally, my t-case shifter box was completely frozen, so i had to take it out and (carefully) beat it with a hammer to get it to move. the rod was especially well frozen inside the splined wheel. but grease it up well and use it from time to time, and it should keep working.