I have a number of electical issues with my excursion:

The following systems are completely, or at least observably inoperative:
Power Windows
Rear Vent Windows
Dome Light
Running Board Lights
Cruise Control
ESOF 4WD

The Back Story:
I bought the vehicle on the first Saturday of November, 2018, knowing it was a bit of a fixer-upper. The windshield was/is cracked, so I scheduled an appointment with SafeLite to get it replaced. They reported that the windshield channel is rusty, and they would not install a new windshield, because of the liability. The windshield also leaked on the driver's side. When I initially investigated these electrical problems, the leaky windshield led me to conclude that the GEM might be bad, because all of these systems involve the GEM, and it is known that a leaky windshield can ruin a GEM. So I bought a part-number-matching replacement GEM on eBay for $100, and spent several hours cleaning and re-sealing the windshield channel. I installed the GEM last night, and was very disappointed to find that precisely nothing about my electrical problems was resolved, or even improved in any way. A couple of contacts in the large, bolt-on connectors on the fuse block had a small amount of green corrosion on them, so I sprayed contact cleaner on both the wiring harness connectors and in the fuse block itself, and scraped the affected contacts as best I could. However, I could see through a gap in the fuse block housing that there is green corrosion on the inside of the unit, which means it should probably also be replaced. Unfortunately, a previous owner scraped the part number sticker off of the fuse block, so I have no way to know what the part number is. The GEM part number is YC3T-14B205-CE, so if anyone knows what fuse block part number goes with that GEM, I'd really appreciate the info. The fuse block has "Y3CT-BF" molded into the side of it, so I suspect that is part of the part number, but obviously it's missing the 6 digits in the middle. I see a number of Y3CT-14A067-BF fuse blocks on eBay for sale, and they look more-or-less exactly like my unit, but I can't be sure they're the right part, so I'm hesitant to drop $120 to try something that's not guaranteed to be the right part.

I found this eBay item, with GEM attached, bearing a similar part number, which may be the right one for my vehicle: https://www.ebay.com/itm/2000-Ford-E...xRT:rk:17:pf:0

Apart from replacing the fuse block, what else might affect all of these systems? A friend suggested checking body grounds, but I'd expect more things (i.e. everything electrical) to be broken if there was a body grounding problem.

Edit:
My ABS light is on, due to the front-left wheel hub going bad. Will this prevent things like cruise control and ESOF 4WD from working?

 

Have you researched the Delayed Accessory Power Relay yet? There is a relay in the instrument cluster that cuts power to some of these devices when it goes out. There are a bunch of posts about this, once you know what to search for. I sent mine out to Circuitboardmedics.com to get mine repaired.

 

In MY2000 the Accessory Delay Relay is Relay 5 (bottom-most) located in the compartment under the steering wheel. They didn't integrate it into the instrument cluster until 2002 (I believe, not positive on the year).

I would start from the simplest solutions and check every fuse/relay that could be related to the electrical issues that you are having and then go from there.

 

I've replaced the relay, with no change in behavior. The radio works without issue (except it doesn't remain on with the doors closed, after the key is turned off), suggesting that the relay isn't the problem.

I have a couple of other potentially relevant issues which I'll add to my original post. Take another look at it in a few minutes.

 

Here's a couple of screenshots from ForScan, showing a bunch of trouble codes related to the GEM.

 

What is your battery voltage at the battery? Voltage at the fuse panel and power port? Both after sitting overnight, and once the engine is running and warmed up.

If you clear those codes, do they come right back?

I think the ABS codes are responsible for the cruise being out.

I would guess your issues to be more intermittent if it was a ground, but checking grounds is never a bad electrical troubleshooting practice and it will reduce the nagging suspicion it could be that - plus it isn't complex and it is free. I'm thinking these codes all have one common factor somewhere - relay, wire, fuse, ground - but my electrical system is different from yours and my manual doesn't cover 2000 so I have no suggestions.