Exactly what does the sticker on the door state? The sticker inside the drivers door usually states a GVWR (gross vehicel weight rating) which is the maximum the truck can weigh with all passengers, cargo and hitch onboard. It will also have a GAWR (gross axle weight rating) for the front and rear axles. This is the max a single axle can carry.
In the owners manual you should find a GCWR (gross combined weight rating) which is specific to the body style, axle ratio and engine in your truck. This weight is the total weight of your loaded truck and your loaded trailer.
You have to be very careful of towing ratings as published by the manufacturers. This weight is based on a base model truck, no cargo, no options, no hitch and only a 150# driver. By the time you add cargo, passengers and a hitch, the max tow rating is reduced pound for pound.
You need to weigh the truck with full fuel, passengers, hitch (add 150# if you don't have one) to get your LVW (laden vehicle weight).
GVWR - LVW = max trailer hitch weight.
GCWR - LVW = max loaded trailer weight.
Never use the dry weight from the trailer literature. Onece loaded with options, cargo, water, batteries, and propane, it can easily weigh 1000# more than the dry weight.
The max you can tow depends on your engine, axle ratio and what your truck weights. Practically, a properly equipped Expedition can tow about a 25 or 26' trailer at about 4500# loaded.
For the size trailer you are looking at, you need a 3/4 ton truck as a minimum. Also, the Expedition has way too short of a wheelbase to handle a 34' trailer.
Sorry to rain on your parade, but I would not go over 25' on an Expy.