There is a seal kit from Carquest- seems like it was around $30. Has everything you MIGHT need to fix any leak. THere is nut on top (think the top one is a jambnut, so there are two). Mark where it is now, and count the turns out as you go, so you know how far to turn it when you reinstall it. I went 180deg too far, and ended up having to get a new box. I would recommend taking it off the truck, and putting Loctite on the threads when you put it back on to make sure they stay on there. This is also a good time to change the fluid, as 90% of it will drain out when you unhook the hydraulic lines anyway.
Daniel