Why does Ford F-150 with an auto box can be driven in reverse and not forward?

Thanks for your prompt reply. I missed to mentioned that there was no oil at all in the pan when I took it off to clean both the filter and the pan. There wasnet any presence of any swarf or bits of metal.I suspect that it could be the TC? Or as you have kindly mentioned the clutcheS? I am dropping the box tomoro and would greatly appreciate if you could tell me what to look for and how easy it would be to replace the clutch rings.I have a very good knowledge as a DIY and know the principles of aouto boxes but never have worked or repaired one nor I am scared to have a go to repair my 2001 FORDY! Ta very much.