[semlin]

Do a search on "part time" and "conversion" and you will learn good stuff. It can be done on a 91-92 with a kit but there is no clean conversion for a 93 onward. You can add a cdl dash switch and manual hubs and remove the front drive shaft. You will gain about 1 to 2 mpg. I believe you will also lose ABS on the brakes and the handling will take some getting used to. Robbie from Sleeoffroad did this as an experiment some time ago.

As for the different types of awd and 4wd, I am not sure what "fulltime 4wd" means as distinct from AWD but I think it means that power is more or less evenly distributed between front and rear all the time, whereas AWD is 95% to the front unless traction is lost. The distinction is a little bit hazy.
Here is an ok article on the subject directed more at cars than trucks.

http://www.difflock.com/offroad/considerations.shtml