CeCe, according to Allen the prime suspect is a worn slip-joint.
http://vehicross.info/forums/showthread.php?t=13741
http://vehicross.info/forums/showthread.php?t=18329
I have not looked into mine yet...the luxury of it not being my DD. But if that is indeed the problem, a driveshaft shop should be able to replace that slip joint for you on the existing driveshaft.