You need to be a little more clear then, in the first post it sounds like you can't get it in N or R, then you say it was slipping, and now won't go anything but forward.
It was slipping at first. Now when i put it in reverse it doesn't go anywhere. When its in neutral it will drive forward. Im not sure if the trannys going bad or what it is. Sorry for not making myself more clear. I just got this truck and the guy had it sitting for 6 months. He said when he parked it that it was fine. Not sure what it could be.