Pulling Castle with 4 wheeler

A suspension lockout is what may be needed.
In many cases it will sag too much. It depends on tongue weight.
You can use a suspension lock like the one listed. I also suspect that a guy could make one with a length of chain or turnbuckle and 2 hooks but I have not tried it.
http://www.northerntool.com/shop/tools/product_200333205_200333205?utm_source=google_PPC&utm_medium=Dynamic+Search+Ads+Test&utm_campaign=DSA&utm_content=_inpage:seats&mkwid=sTkQ5PIBw&pcrid=81459274751&mtype=b&devicetype=c&storeId=6970&langId=-1&type=search&gclid=CNzhpoOTk8oCFQuLaQod5BsEig

I seen guys build a spring wheel for the tong of the trailer to take the weight

This looks pretty easy to build

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May want some tong weight for traction.