Hello. I have a 06 4-runner w/the big 6 cyl. I have a 16 ft, trailer that I tow the atv's with. It doesen't matter if the trailer is loaded or empty, when I'm driving at the speed from 40-50 mph the 4-runner has a slight shake to it, [ not above or below those speeds]. The runner has just reciently gotten 4 new tires and front end alaignment, but no change, it did the same thing before the new ties and alainment. Does anybody have any ideas as to what could be causing this. At approx, 30k miles [runner now has 60k]I had the tranny fluids and filter changed over to amsoil, at that time the tech said the tranny is in exel, shape and wouldn't need to be changed till 90k. Runner pulls fine, no probs braking, steering, up hills -down hills, just towing between 40-50 mph. Thanks ahead Leon.

If it only happens when towing the trailer loaded or unloaded. This is harmonic my guess is one of your trailer tires has a slipped belt. Not a big deal as it will go away pre speed and post speed. It would suck if the speed limit was in that area. Put new tires and balance the trailer tires. 99 percent sure thats your problem. I sell trailer hitches but have been in auto repair for 15 years. The fact that it starts at one point and stops at one point is a dead give away I am using deductive reasoning that 4 new tires and alignment it has to be the trailer tires. For what it worth.
Thanks I can be emailed at 5solson2@ gmail.com or scott@hitchheaven.net.