When we first got our 2005 Fleetwood Excursion 39S we also had slide problems with the left side. The Excursion is basically the same as the Expedition except it has a higher ceiling and some upgraded do-dads.
The left side as you've seen has to come up over the hump which makes it harder to retract. Using slide lube underneath helped a lot. The other part of our problem is the batteries we got it with weren't the best. If you start the generator it will put 60 amps into the batteries and that brought the slide in without a problem.
Bill