I have a 1997 Ford F-150 XLT 4x4 with the 4.6W and a 4R70W trans. For the past year is has been leaking trans fluid and has slowly got worse and worse. At first the truck would only leak at high speeds (when driving on the interstate for extended periods of time) and now It has gotten to the point where I am adding close to a quart of fluid a week. ( roughly about a quart every 300 miles!). I thought that it was the pan gasket leaking at first, so I flushed the trans fluid (dropped the pan and drained the TC and put on a new filter) put a new gasket on and torqued it all down and added some new Valvoline max life fluid. The first trip in the truck I noticed a puddle of fluid under the truck again. The pan looked like it could have been a little warped, so I ordered a new pan and a new FoMoCo gasket (the one the came with the new filter was the cheap felt type). I thought I for sure had fixed the truck this time, but I drove it and once again it was still leaking. I checked all that I could before replacing the pan and gasket. I checked the trans cooler lines and fittings (no leaks) I pulled the inspection plug and it was dry (so I ruled out the front pump or TC leaking) I have run out of ideas. The whole underside of the truck is covered with fluid so it is hard to pin point the leak, but most of the fluid drips down on the transmission cross member. I'm sorry for the long description, but I wanted to provide all the info I could. I anyone has any ideas or suggestions it would be greatly appreciated. Thanks!

 

I would clean the trans off, including the top of it, then drive it around and check all over often. Want to be able to see where it is leaking from, before it leaks so much that it is all over everything in a big mess.

Some thoughts:
Trans case crack
Gasket from trans case to transfer case
ANY item that passes through the case, like manual valve shaft, speed sensor, wiring harness, dipstick tube to case, pipe plugs on case, etc. etc.

It's going to take a lot of looking, often, to find it. At least being a truck, you can get under it to check. Not like a FWD car!

 

Did you ever figure out the leak?

 

Transfer case was loose where it bolted to the transmission. I tightened the bolts and never had a drip since. I have, however, sold the truck and no longer own it. But up until December 2016 (when I sold it), the trans remained sealed with 180k on it.