Part number AS260 small bottle AS261 large bottle for the fluid at Autozone.
Here's A-Z's diagram...
Power Steering Pump | 2001 Ford Expedition 2WD 8 Cylinders L 5.4L EFI | AutoZone.com ...its basic but better than nothing. Your simply going to have to climb under and look.
I would suspect either the shaft seal or the line connection at the pump base. But it is possible your hose could be rubbing on a pulley. Where the lines run from the base of the pump up through by the frame there should be a wire loom retainer for those lines to keep them from the pulleys. I caught mine just in time greasing up the front. All you need to do is bend it away from the pulleys. But in your case you may need a new line which you can get for about $20 at most auto stores.