reverse won't work....

I have a 88' beretta with a 3.1 swapped in. I just got the car and when I put it in reverse, I can feel a VERY light engage, but then it's as if it's in neutral, it will rev, but not move anywhere or even try. All the other gears work fine. I was told to check the trans fluid and possibly change the tranny filter. I do have some tranny stuff called "Trans X" to stop slippage, but I haven't used it yet (was told to use GM stuff instead). What could be the problem? I have posted on other forums, but i'm looking for all the help I can get. Thanks for any help.