Thanks for all the help guys! I've been doing a bunch of research and I think I've decided to slap a diesel and F-250 running gear into a Bronco (same front clip as the old square-body F-250's, some even come with the 7.3's tranny) and then I'll tub out the Expedition and make it into a desert runner. It would just take too much work to make the Expy a diesel.
Oh, and in case anyone was curious, I found out that Ford had planned to try the 4.5L V-6 Powerstroke out in 2005 Expy's and F-150's and decided there wasn't enough demand, then did the same exact thing with the 4.4L V-8 diesel they were thinking about putting into the 2011's. I'll be keeping my fingers crossed and hope that the third time's the charm. Hopefully it won't take too long!
Thanks again for all the help!