07 PF, how much drop on hitch receiver?
 

I have an '07 Pathfinder. I recently put a 4" drop Master Lock hitch receiver to tow a Bayliner 185. Sometimes while towing at about 70 mph, the whole truck starts to shake. I'm wondering if the tongue weight is too low, and perhaps I need to go with a 5.25" drop on the hitch? The back end of the trailer does not seem to be moving side to side, it's almost a front to back motion, like its about the stall, but the engine tach. and speed are fine. I'm not using O/D.