Transmission problem with reverse

I have a 1994 GMC Sonoma,Automatic, 2WD, 4.3L, Vortec V6. When I put it in reverse it will barely move. You can feel it go into reverse, but it only rolls backwards. We have checked the fluid, and it's fine. It also shifts just fine while driving it. Everything seems to be fine except for the reverse. What could be the problem? Any information would be greatly appreciated. Thanks.