I had a terrible porpoising problem. The ultimate fix was to install better front shocks and to add a leaf to the front springs. Apparently this is counter to engineering beliefs, but the shop I went to had just completed a similar fix for a fire truck..having had many discussions with the frame manufacturer, who dissed the idea...it worked for the fire truck and it worked on mine.
The problem is an engineering circumstance where the energy created at the front of the vehicle is not being transferred properly down the frame and out the rear. Adding the extra leaf and then stiffening with the shock, corrected that. And believe me...when I porpoised...my front would lift right off the ground, seemed like.
I can find my pix of you need them.
Gary Haupt