Presuming gearbox = transfer case
and that one of the cross members is the transfer case/transmission skid plate and the other is the actual cross member that the transmission is mounted on.
there is a paper gasket seal that goes between the transmission and transfer case, unless the transmission is grossly overfilled then this should not see more than a splash and will not leak unless the transfer case is loose because of the machined lips that are present to ensure centered mating of the transfer case.
The rear tail housing can break through you should have the cast iron tail housing which is the strongest the factory offered. and of course there is a gasket there also but not one that is known to leak unless bolts get loosened. BTW the torque specs are not very high due to the fact that the case is rather easy to strip out. If you have a newer transmission that was replaced as a swap in then it is possible that a sub-par core was used which could result in these problems
Based on where you are seeing the fluid it could be any number of places that are leaking. The most common is the pan gasket or possibly the drain plug if someone did not torque the pan down right when changing the filter or worse that they stripped out the pan bolts when tightening down the pan, the good news is that if this is your problem it is easy to put thread inserts in these holes or simply use longer bolts with the proper tap since ford did not tap the holes all the way through the length that they drilled (this works on all except a few of the front and rear holes since they are blind holes)
Is the pan wet with fluid? what about the inspection cover at the front of the transmission?
