Did I ever work out the numbers when I bought my TT (2012 Kodiak 263RLS), and confirm that my '04 Expedition EB could tow it safely... and just like my setup, you will hit the limits of the Expy. You have a similar TT length and weight-wise as mine:
Dry Weight (lbs) 5783
Payload Capacity (lbs) 1817
Hitch Weight (lbs) 670
Your Expy MY is much better suited than mine (engine and tranny), but as others have mentioned the Expy has 2 weight limits: RGAWR and payload (knowing that you can fit 8 passengers - be careful there). Loading the truck with my 3 kids and the TT hooked up:
TV payload 1320 Lbs (max. 1420 Lbs)
Rear axle weight 4050 Lbs (RGAWR 4128 Lbs)
TW 820 Lbs (max. 920 Lbs)
TV loaded weight 6200 Lbs (GVW 7600 Lbs)
These weights are as measured at a CAT scale while heading out for our first camping trip last year, and the TW was measured using a Sherline scale.
I'm pushing the limits of the truck with this TT, and I know it. But with the right equipment installed and set up (Reese WDH, Reese Dual Cam, Load E tires on the Expy), the rig handles the road quite nicely: 1/2" rear end squat / level front end, no sway, crosswinds are not a concern... With over 2000 miles since our first ride last year, the setup makes for a relaxed-knuckle ride.
Enjoy your setup.