I have the same trans with a different problem: If I park my truck facing down my steep driveway for a day or a month, when I start it and let it warm up it goes right into reverse and once up the driveway, and put into "drive", it pulls away perfectly and shifts perfectly. However, if I back it...
This is a very dumb questions I know, but I did see it once many years ago: Do you have an after-market dip stick?
Have you done a complete oil change and filled with exactly the amount of oil specified for your engine?
Do you have an after-market oil pan?
You can see where I am going with this...
Or, if you want to try something that will not hurt anything, put in the large bottle of Q-Seal and follow the directions. My head gasket ('96 7.4L Gas) was leaking bad and thought I would need to replace it or rebuild engine. That was 26K miles ago. I put a new bottle of Q-Seal in with each...