97 Expedition died on side of road.

Have the battery checked out. If its good check all the belts to make sure there is no slop in them. Jump starting is an easy way and a quick fix but doesn't slove anything.

I think I have a very similar problem. I hope the additional details might help:
Our 1998 Ford Expedition EB (5.4L 4x4 towing package) just came out of routine service on friday, and ran great over the weekend, but I haven't been able to start it since I pulled-up in the driveway monday.
When I first tried, all electrical power went out as soon as I switched the key to start the engine. No dash lights, no hood light, nothing.
I was hoping something obvious would be disconnected under the hood (that might have been loosely reconnected after the service, and fell out from the vibrations of driving it over the weekend). No such luck, though, so I disconnected the battery and verified it did have voltage, then glanced around for blown fuses, wiped the terminals and reconnected the battery.
I immediately noticed the under-hood light came on, so I tried the key, and again as soon as I got into the start, all electrical power went out.
I again disconnected the battery and looked around some more. I did notice there was a gray connector box (with a bunch of wires on both sides) mounted on a metal bracket track off the firewall that slid freely a couple of inches or so, so I pushed it back until the tab locked in place. There were also a couple of smaller black connector multi-wire boxes mounted in front of the same metal bracket, but they seemed to be mounted on tight.
I saw a small grey fuse-looking thing with some terminating a wire within one of those bundles in the very corner, I pulled it out and cleaned some of the corosion (in outside cavities, not on the connectors) and replaced it, and again after wiping and reconnecting the battery, the under-hood light came back on.
Wondering whether the ignition was the problem, I did not use the key this time, but instead, got in the car and tried to put a load by turning on the headlights, and again, lost all electrical power immediately.
I called my mechanic to tell him that I just hated the idea of getting the car out on friday, and having to tow it back this week for something that could be a very trivial fix. Reading this and a couple of other posting boards, I wonder whether the problem might be the starter relay solenoid (mounted on the firewall) or neutral-safety switch, or ignition switch, or the anti-theft kill-switch misbehaving, although that last one might be far-fetched.
Again, I haven't only lost the ability to crank the engine, but all electrical, and it seems to come back on everytime I disconnect the battery for a few minutes, but only for the low-draw devices, like the under-hood light, or dash-lights when I put the key to on, but NOT start. I left the battery connected this time to see if something would reset by itself, but I haven't gotten any power back for a couple of days now.
I bought the Haynes for this truck and the general Haynes electrical manual. I'm no mechanic, but I was hoping there might be a methodical set of steps you could recommend to rule-out the very obvious things, starting at the battery and working my way up the various harnesses (I have a cheap analog multi-meter).
My wife stays home, the school is within walking distance, I have another car I can drive to work, so I do have a few days to chase the obvious before deciding to tow it back to my mechanic, if it is a more complicated electrical problem.