[MrBasse]

I made my earlier comment assuming that you already had your frame solid. I strongly recommend following the advice of everyone else in this thread first. Cross bracing and a belly pan on your frame is going to make the biggest difference.

If you are still concerned with impact, reinforce your bumpers. They are designed to transmit forces more evenly to the frame. The frame is still going to take a hit, but it should be distributed better by the bumpers.