DatGuyC is right on the money. Image loaded for me. It is the torque converter shield drain port. If you're 100% that it's ATF then DatGuyC is again right on both counts: your torque converter is leaking or the transmission input seal is leaking. Both require the transmission to be dropped from the HMMWV and to inspect the components. If it's the torque converter, then it can easily be replaced, the transmission reinstalled, and go right on your way. If it's the input seal, I'm fairly certain that the transmission will have to be disassembled in order to replace that seal, which isn't for the faint of heart, but could be done with the right tools, time, clean environment, and patience....or money. It's an ordinary TH400/3L80, so every transmission shop in the world should be able to repair it.