Further to what Snokaw wrote about using a sheet of plywood...
I'll try to describe this, but if I get the chance, I'll post pics which will help alot..
If you put a piece of plywood, say 3/4" or even 1/2" in the back, but screw some 8' - 2"x4"s to the bottom, along the edge of the plywood, that will give you a bit more support. Now to take it one step further, and this is what I did - go to your local hardware store and buy 2 eye bolts, 2 lengths of chain (probably 5 links each), and 2 turn buckles (one eye end and one hook end). You will also need 4 of those chain links that are used to connect 2 lengths of chain. You put the chain through and squish em closed..
Anyway, screw the eye bolt into the corner of the plywood, through and into the 2x4. Connect the chain link to the eye bolt and length of chain. Use the second chain link to connect the length of chain and the turn buckle.
At this point, you should have an eye bolt stuck in your plywood. Connected to it is the chain and the turnbuckle. The unused end of the turnbuckle is the hook end. If you look on the inside (side wall) of the box of your pickup, you will see a hole near the tail gate latch pins. You can put the turnbuckle hook into there and then tighten it up to the point where the plywood just starts to lift off the tailgate.
Now, when you load your sled, you are distributing the weight between the plywood and 2x4's, those loaded turnbuckles and your tailgate.
This would have been so much easier to post a pic, but hopefully what I wrote makes sense. Send me a message or reply here if you would like clarification.