Transmission Problems with a 4WD 7.3 Automatic
 

I am needing some advice before i start throwing down money to get my truck back in working order. It started a couple of weeks ago when the ms. was driving the truck. She was running into town and it shifted normally through all the gears but when she got around 60-65 it downshifted and reshifted up a couple of times without any change in speed and was running down a straight flat road. I was informed and checked fluids which all appeared to be fine and didnt think much else of it until i got a call from the teenage driver in the house saying that the truck will not move. From what i was told he was driving it "normally" and then it began to smell like smoke and he pulled it over. Just so happened that he was at his destination. I went and inspected the truck and it appears as though it has blown transmission fluid out of the dipstick tube and all on the underside of the hood and would not move when in any gear. It took me 7-8 quarts to get it back up to the proper level of fluid. I had it running and had him shift through the gears with me underneath it and i could not see where anything was leaking out. Now the truck will go into reverse with no problem and will move when in 1st or 2nd but if you try OD it acts like it is stuck in neutral. I dont want to drive the truck home and risk further damage but what could be causing it to not work in OD? Sorry if this is long but i wanted to be as detailed as possible. Thanks for any help...