I don't know the specific history. But there are several articles in the "How To Tech" section of this site that refer to the PVH (pulse vacuum hubs) system being used from 1998 to 2000. So, if you don't have little turn knobs on your hubs to engage or disengage them manually, then you most probably have vacuum hubs.
The Pro for PVH is that if the system is working, you don't have to get out of the vehicle to manually engage or disengage the hubs when you want to use 4wd. The Con is that the system doesn't seem to be very reliable, probably due to potential vacuum leaks or problems from dirt, corrosion or lack of lubrication.
Follow the "How To Tech" link at the top of the page and you can learn a good bit about the PVH system and options to get around it.