Are you looking for trailers that can transport multiple pallets or a single unit? We had two trailers custom built to transport eight pallets per trailer. They were pintle hitch and capable of 20K pounds or so. They're not four wheel steer, but the front wheels are articulated and they can be hitched together. I don't recall the manufacturer, but I can check when I get back to the plant.
As a matter of fact, these trailers are no longer in use.
JC