Yep search on "Porpoising."
We were dealing with this with our 2012 F150. With advice from the forums and my WD hitch manufacturer I a) lowered the nose of the trailer a bit b) got the inflation on all my tires exactly right and c) dialed up the WD on my hitch. It is much better. I have been considering, on the recommendation of others, firming up the suspension with either Bilstein shocks or the Roadmaster Active Suspension (leaf-spring helper). For now I think it is solved so I'm staying with the stock set up which now other than the worst roads drives very nicely.
I found Paula Ford's post #7 on this thread very helpful: