A slip yoke eliminator elimiates the slip yoke <IMG SRC="/board/images/smilies/dunno.gif">
Actually the rear driveshaft in stock form has a end that slides into the tranny/t-case output. When the axle moves up and down, it lets the driveshaft lengthen and shorten at the tranny. Unfortunately if you over extend the slip yoke, it can fall out. It is also not as strong as a slip-yoke in the middle of the driveshaft.
An eliminator will convert the tranny/t-case to a fixed yoke (looks like the pinion yoke on an axle). This prevents the top end of the driveshaft from falling out. You should replace the driveshaft with one with a long travel slip system in the middle.