2002 f350 w/ 7.3l, 4x4,extended cab long box..... Drove for 1/2 hour, went to back up, wouldnt go, noticed tranny temp guage almost to the yellow, still go's in drive, I checked the tranny fluid, was very low, added 3 litres still no reverse, no leaks under truck, when i put it in rev. It revs up as if it was in gear but no go, very confused! Plz help