Intermittent Vibration at Highway speed with manual transmission

It's a completely stock minus the borla cutback exhaust 2020 trd sport access cab 4x4 m/t it's at 12k miles but I've had the vibration that is felt in the seat and shakes the entire passenger seat that starts faintly 45mph and continues up past 85mpg with it getting worse the faster you go and the weird this is it seems to be intermittent where the vibration gets worse then gets better and worse then better then almost goes away completely around and around again will cruising at the exact same speeds. I've had this happening since 1k and I took it to the dealer at the 5k service and mentioned it to the service writer and he said that's normal and his taco does it to. They had a tech balance the tires and rotate them and look it over but could find anything and I even took it out on a drive with the tech but wasn't able to get it up any fast the 60 because of where the dealership is and he says it was one of the better ones he's been in. And they just chalk it up to that's just the way they are which to me being a mechanical engineer does NOT make sense at all. It's now in for its 10k service and I mentioned it again and they are going to look at it again. I just can't accept that they are supposed to shake like this.
sometimes I look in the side mirrors while it shaking and it almost seems like you can see the bed shaking to
Has anyone had a similar issue if so were they able to fix it and if so how did they do it?

 

I have the same thing. Been doing it since new and has been getting worse. I have about 20k miles now. I’m honestly thinking of trading the truck in. I have a 08 FJ with 248k miles that I’ve had since new without any real issues and is the main reason I bought the taco. But between the vibrations, stupid drum brakes, horrible engine responsiveness and other small gripes, I don’t think I want this truck for the long haul.

 

First, break loose your lugs and retorque by hand in the proper criss cross pattern. Just to make sure that isn't an issue. If that doesn't help,

Find where the local hot rod crowd or autox crowd hangs out. Ask around about the best shops for tire service issues. One with a Hunter Road Force machine.

Take it there and ask them to find if it has an out of round tire or one with a broken belt. If you have a bad one (or two) your options are limited to whatever the OE tire warranty was. At 12k that may not be much.

If the tires are healthy ask them to balance the tires, and do the Road Force procedure on any that are out of range using the standard method.

If the right tech does it the right way, and the problem still exists, you'll have at least eliminated tires/wheels as your problem.

And no, they don't 'all do it'. You should also look for a new dealer to do any free/warranty work, and a quality indy shop for other maintenance.

 

Curious if you ever found a solution? I have a similar shaking starting at 60mph and up.