I haven't flat towed one before, but I can tell you that the factory tow hook location in the frame is strong enough to pull the truck and a 16KLbs. 5er out of axle deep sand. So any towbar setup that would use those points should do the trick.
The 200 HP 4WD John Deere couldn't do it, but the D9 Caterpillar dozer did. I expected the tow hook to shear from the frame, so I went as slow as possible to take up the slack and pull. Dozers are nice, as they have low gears and decels. I have to post the pic, but I currently cant find it.
2000 Monaco Windsor 40 PBSS