It is not uncommon for the 4R100 transmission to leak out of the front pump seal, especially at times when it is getting hot. In some cases, removing and replacing the front pump and seal is helpful, if this is confirmed as the leak source. But, in others, the seal leaking is merely a symptom of a bigger internal problem which is causing an overheating transmission.
A diagnosis stratigy would be as follows. 1-Time at which leak occurs(hot/cold/towing/idling) 2- check temp at cooler inlet at trans. 3- Confirm correct operation of transmission and pan inspection. 4- Remove trans, inspect seal and hub bushing. 5- If hub and bushing ok, replace hub seal(probably, failed grey national seal).