Your 8500 lb towing capacity is calculated without any aftermarket accessories, cargo (in the truck), or passengers. in the vehicle. As you add weight with people, accessories, and cargo, that 8500 tow capacity is dropping, pound for pound. If you put 1000 lbs of people and stuff in the truck, it's max tow capacity is only 7500 lbs.
Besides a max tow rating, your truck also has a rating for what it can carry. It can't carry it's max payload and tow it's max tow rating at the same time. That is because part of that max trailer weight is being carried, not towed. Look for the tire / loading sticker on your drivers door post. It will show a max CCC / passenger number. That is your payload (what the truck is rated to carry). That weight capacity is eaten up by people, pets, cargo (in the truck), weight distributing hitch, and trailer tongue weight. Overloading the tow vehicle can also have an impact on trailer sway.
Example:
Say the tow vehicle has 8500 lb tow rating and 1500 lb payload.
The 8500 lb trailer would eat close to 1200 lbs of that payload. The WD hitch would eat another 100 lbs. That would only leave 200 lb of payload to carry the driver, passengers, and cargo.
Check your truck tires for both air pressure ratings and the type of tire. Passenger (P) tires have softer sidewalls and lower air pressures than light truck (LT) tires. Softer sidewalls allow more side to side flexing. Good for passenger comfort, bad for towing heavy loads.
If you've got the hitch setup to tow the trailer nose-up, not enough tongue weight, overloaded tow vehicle, and passenger tires that are not inflated to their max pressure, that's a perfect recipe for sway issues.
18 Nissan Titan XD
12 Flagstaff 831FKBSS
Wife and I
Retired Navy Master Chief (retired since 1995)