The 5.4 3V came out with 300hp in 2004. The 4.6L was put to pasture late 2003 due to the weight increase of the new 2003 model, so the power to weight ratio suffered. Earlier Ex's such as your '98 can have anywhere from 215-230hp depending on engine. The 4.6L had 215hp and the 5.4L came standard with 230 for 1998.
Check this out: 1998 Ford Expedition Engines and Fuel Economy - MSN Autos
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!