Suggest cleaning the area and confirming the leak location. If between the flange and the diff, then the pinion seal is leaking.
There is a Ford TSB regarding oil migration through the threads of the Pinion bolt. The fix is to install RTV silcone between the pinion nut, pinion flange, and pinion shaft. Remember anytime the pinion nut is removed, then the pinion bearing pre-load has to be re-established.
Note, the design of the diff causes the oil to be pumped against the front pinion bearing. In fact, if you look closely at the inside of the diff, there is a channel above and below the front pinion bearing to allowing the bearing to be bathed in an oil flow.
I agree with trinity_gt, the oil slinger will tend to direct the oil away from the pinion seal during operation. The fact it does not leak when setting level may indicate the fluid level has dropped below the level of the leak.
Remember that the proper fill level is 1/4" - 9/16" below the fill hole. It is not fill until it runs out.
My vote is the pinion seal. If it were thread migration, it would be a small slow leak all the time. Where as a sudden larger leak when the car is tilted forward would indicate the pinion seal is the source.
FWIIW; I had the same problem on my 2003 GT. I ended up installing a new pinion seal and RTV on the nut. It was a lot of work. Recommend doing your research before attempting yourself for the first time.