I agree; clean off the steering box so you can locate the leak source. While under there find the numbers on the steering box casting, that number and perhaps a brand name is very important for locating a new box. This is nothing special, virtually all are heavy-duty truck parts.
I will say it is probably a leak from a power steering hose or fitting instead of the actual box, but when the box is oily/greasy it's impossible to accurately locate the source. Use engine de-greaser spray on the box and surrounding area then flush with water. If it has air suspension, do not get underneath without the frame being blocked so it cannot lower!!!
Some brands are rebuildable, some are not, so if one must be ordered, make sure the shop has received it before removing the old one.