hey buddy...i don't want to sound like a complete douche, but you'll learn a lot by using the search feature on this site...
but the point is, right now you have a slip yoke...meaning if you're under enough flex, your drive shaft will more or less 'slip' out...
with a slip yoke eliminator, you 'eliminate' the possibility of such an occurance...
the transfer case drop is provided as a cheap way to prevent drive line vibrations...if you decide to go to an SYE, you are required to switch to a CV driveshaft, thus eliminating the need for the transfer case drop...
read up on the subject, i've only skimmed the surface...