reverse not working

I have a 1997 Jeep Grand Cherokee Limited. The past couple days reverse felt weaker and weaker, less and less moving, with a revving engine. Today, reverse doesnt work at all, it acts like neutral. I hear no slipping of gears, but if im on a completely flat surface, it will crawl just a little as soon as i get it into the higher RPMs.
Does anyone know what could be wrong? How could i fix it? This is also the second transmission on my Jeep.
-Mike