The steering isn't loose -- you've pulled too much weight off the front axle. Truck tires, excessive tire pressure, and insufficient weight distribution (transfer to the front axle) are the problems. Unless you scrap the truck tires you can only fix two out of three but it should work.
The Expedition can support the maximum allowed rear axle weight at the tire pressure listed in the door jam which I recall is 36psi your sticker will have the pressure. Truck tires aren't necessary as the P-Metrics are more than capable of handling the allowed weights. I run my P-Metrics at 40psi when towing -- when I remember -- just to take a little squirm out of the load but even at 36 there's no sway/fishtailing 'cuz my WDH puts weight back on the front axle.
An 8,000 pound trailer requires 10% to 15% of that weight on the ball for safe towing -- 800 to 1200 pounds. Even the minimum of 800 pounds will unload the front (steering) axle and unless your WDH puts weight back on it you'll get loose steering. Note the maximum weight on the ball is 920 pounds and that requires a properly adjusted WDH.
Recommend weighing the rig and comparing the weights to the allowable axle ratings. Also note the cargo capacity of your Expedition. Mine is roughly 1450 pounds and if I put the maximum allowabe weight on the ball (in my case 920 pounds) I'd only have 530 pounds for everything else. With my 120 pound former swimsuit model wife on board that's just 410 pounds remaining for everything including me and the dogs.
Overinflation severely decreases the tire contact with the road.
-- Chuck
