Like he said, check your fluid level, that's always the first thing to do. Without knowing more specifics, other possibilities could include a linkage problem, or a problem with the reverse band inside. Do you have any hesitation in the forward gears? If you pull it all the way back into 1, does it grab right away and hold it until you shift to 2, and is there any hesitation? If you are driving slowly ~5mph, pull it back into 1 and then put it into 2, and just idle along going back and forth, this should tell you if you have clutch problems rather than just a bad band. And if you have hesitation everywhere and your fluid level is ok, it may just be a plugged trans filter. These are just common tranny diagnosis things, some other people may have more specific advice.
1994 ZJ, Limited, 5.2L
Black, Of Course