Just got back from round two of the summer trips. This time a 1600 mile round trip to Woodward, PA to pick up my son from camp. I noticed a whine from the transmission when I took off from a light so I pulled into the gas station and checked and no fluid on the dipstick. This is a 2000 2wd XLT with a 5.4 in it. It was not slipping, did have any issue changing gears and I have no fluid in the antifreeze, on the ground, or on the undercarrige. It is going somewhere. I was told to look for white smoke at start up and acceleration and I see none of that but that does not mean it is not coming out when I'm driving or is very faint. I had to add about a quart leaving PA yesterday morning and now that I'm back I heard the whine again pulling into the driveway. I'm sure I'm a quart low again.

 

Maybe it's coming from on of your tranny cooler lines up front. It would only leak when you are driving. Worth a check, happened to me but then I saw a few drips on the driveway.

 

Ditto, the leak may only be presenting on the front of the truck. My cooler line leak was at the fitting going into the cooler. It splattered all over the front suspension but never got back to anywhere near the transmission to make me think "gee there's a transmission leak". Check the front of the truck.