Leveling Pickup bed to the hitch rails

Are you intending to put rubber pads between the hitch and the bottom of the truck bed?! I predict stretched and broken mounting bolts in your future at the minimum. But if you want to persist in this scheme Tractor Supply sells horse stall rubber pads that appear to be about an inch or so thick. Not big enough to cover the full bed, I'd guess, IIRC, that they may be 4 ft square. Look up their website.

But you could probably reproduce the effect just by stacking an inch or more of flat washers under the hitch around the mounting bolts. Let us know how it goes.