Most tailgates are "RATED" for between 200 and 300 pounds depending on the truck. I would expect the superdutys to be 300 pounds.

That said I have put 400lbs+ on my tailgate many many many many times.

Also, I have hauled motercycles, quads, buggies, you name it, all using ramps on the tailgate, you will be fine. Just be sure to secure the ramps to your bumper with a ratcheting strap.