I picked up my front driveshaft this afternoon from the same place I got the first one, Denver Truck Parts (38th and Walnut for us Mile Highers). The guys there were really helpful. Normally they have a policy of "check your part before you leave, if you buy the wrong part, you now have a spare" (as the sign reads). But I explained to them what I was doing and that my driveshaft was too long. So they took me back to the warehouse and sized it up along with other F-150 front shafts. Sure enough, I found the one I needed. The clerk said "Yep, that's the 4.6 liter." I'm going to put this in bold so people can find it later:
The front driveshaft needs to come off of a 4.6L V8 F-150 or Expedition.
For the same reason that the rear shaft needs to come from a 5.4L V8. The transmissions used on those trucks are different lengths (with adapters). Therefore so are the driveshafts. The 5.4L front shaft is about 4 inches too long for the Explorer with the BW 4406 t-case. I guess all the rest of you got lucky and picked up the correct shaft. Anyway, my hybrid u-joint was already on the other shaft, and I asked if they would switch it out for me. They were too busy, so he told me to just take it. So I got my new driveshaft for free!

Two for the price of one, except that one of them is essentially worthless.
I will be installing the new shaft tomorrow afternoon most likely, and will then search for some stuff to climb over in 4LO!
If anybody wants a spare driveshaft for their 5.4L F-150 or Expedition, let me know. That goes for the perfectly good Explorer AWD transfer case as well.