Cut and pasted from another thread on a similar subject:
"We have towed our 19-foot Bambi from Phoenix to the SF Bay Area numerous times with both a 3/4 ton Chevy crewcab with a long wheelbase and a 1/2 ton Tundra CrewMax with a short bed. The stretch of I-10 between Indio and LA is really bad with harmonic dips that get both rigs bucking like a bronco. I don't think switching tow vehicles will help much; the real problem is the roadway. We now take some parallel side highways that are much smoother to avoid this stretch."
There is a lot of discussion on truck forums, too. It's not the tow vehicle suspension or shocks, or the hitch, or the weight distribution in the trailer; it's the roadway. The old concrete slabs sag in the middle and pitch up at the joints; and when the frequency (speed) of the tow vehicle and trailer match the length between the joints, random bouncing becomes harmonic porpoising. The only way to reduce this effect is to speed up or slow down to reduce the harmonic oscillation. This will not stop the bouncing, only reduce the magnifying effect. Or, use another route until road crews grind down the high spots at the joints or repave with asphalt.