Is the 3.31 even an option for the 4x4? I checked the fleet site because it usually has more info and the regular site was hanging up on the '08 Specs.

Here's a copy/paste from the fleet site.
https://www.fleet.ford.com/showroom/2008fleetshowroom/2008-Expedition-specs.asp?EV=0
<TABLE id=tablegeneral cellSpacing=0 cellPadding=4 border=1><TBODY><TR><TD vAlign=top colSpan=6>Maximum Loaded Trailer Weight Ratings (lbs.) </TD></TR><TR><TD vAlign=top>Axle Ratio </TD><TD vAlign=top colSpan=4>5.4L V8 </TD></TR><TR><TD vAlign=top>3.31 </TD><TD vAlign=top>4x2 Class IV </TD><TD vAlign=top>6000 </TD></TR><TR><TD vAlign=top></TD><TD vAlign=top>4x2 Class IV </TD><TD vAlign=top>9200** </TD></TR><TR><TD vAlign=top>3.73</TD><TD vAlign=top>4x4 Class IV </TD><TD vAlign=top>6000 </TD></TR><TR><TD vAlign=top></TD><TD vAlign=top>4x4 Class IV </TD><TD vAlign=top>9000** </TD></TR><TR><TD vAlign=top>Engine </TD><TD vAlign=top>Horsepower (hp @ rpm) </TD><TD vAlign=top colSpan=3>Torque (lb.-ft. @ rpm) </TD></TR><TR><TD vAlign=top>5.4L 3-VALVE TRI TON V8 </TD><TD vAlign=top>300 @ 5000 </TD><TD vAlign=top colSpan=3>365 @ 3750 </TD></TR></TBODY></TABLE>
I don't see the 3.31 4x4 option here.
*********** EDIT **********
Right after posting, I checked the towing guide and it shows the 3.31 as an option for 4x4 (non-EL) and a 15,000 GCWR for 3.31 + HD tow package
https://www.fleet.ford.com/showroom/rv_trailer_towing/2008/08RVttscltrp23Oct1907.pdf